Before the days of MIDI sequencers, and keyboards with realistic-sounding samples of instruments, in order to even hear what one of my arrangements sounded like, I had to assemble all of the musicians required to perform the piece, and get them to play it for me. If I didn't have access to a multi-track tape recorder, and a mixing console, etc, that meant that I had to get them all together simultaneously to play the piece live in order to hear the full arrangement. If one of them couldn't show up at a scheduled time (and musicians are notorious for not being able to stick to a schedule), then it was tough luck for me. I had to wait until the guy finally got around to playing his part so that I could actually hear what my arrangment really sounded like. In the meantime, I had to twiddle my thumbs. And how else can a composer know if his arrangement is what he wants until he hears it? Of course, if I decided to rewrite the parts after hearing the arrangement actually played, that meant that I had to reassemble the musicians again at a later date to hear the new arrangement (and usually, they don't want to have to redo something unless you're paying them, which can be expensive, especially if you need to "motivate" them with cocaine).
MIDI/computers make it possible for a composer to hear his arrangement played without needing other musicians. It thereby frees the composer from scheduling problems, and allows him more opportunity to audition his composition during the composition process, which provides invaluable feedback to aid in composition.
I've still got a stack of arrangements sitting around from years back which I've never even heard because I can't find the musicians to play the scores. The music I write often requires some difficult technique. You know how hard it is to find musicians who can play this stuff in a small town like the one in which I live? Almost impossible. And that's assuming that the musicians in this area even want to play the music I've written. I write "progressive rock" which is not all that popular music. Most musicians prefer to play music they like, and that would be "popular music". I don't write pop music. So it's doubly hard for me to find musicians since I write difficult-to-play, esoteric music.
MIDI/computers made it easier for composers to hear difficult-to-execute, or esoteric, music. It is often easier to have a computer play a difficult performance than it is to find a musician capable of playing that performance. Likewise, it is often easier to have a computer play esoteric music (ie, unusual instrumentation) than find musicians who have appropriate skills and the desire to play that music.
And even if you could get all of the musicians, the time it took to do a multi-track recording on the old analog equipment, what with setting up microphones, running cables, rewinding the tape, etc, takes so much longer then pressing the record button on an electronic keyboard with a built-in sequencer and its own samples of instruments, and immediately playing your part. You write a long piece (such as my 8 minute long "Joan of Arc" arrangment on my web site) and you can spend minutes just waiting for the tape to rewind to hear just one of what will likely be hundreds of "takes" to build up the parts. And don't forget that with analog tape, you can't zero on an individual musical event and "correct" the wrong note just by entering the "correct" MIDI digital value like you can with a sequencer. If you make a mistake, you have erase over it, and literally play it again. To save time, we did "punch ins" (ie, recording over the area on the tape only where the mistake occurs), but this is hard to do. If the guy pressing the record button didn't happen to press it at just the perfect moment (ie, we didn't have a computer to automatically punch in at a certain time -- we had to do it manually -- so the musician playing the overdub always needed someone else around to press buttons), you could miss the attack on a note, or destroy the release on a previously recorded note that you didn't want to record over. And if you blew it, then you had to back up the tape and punch in earlier (ie, each time you made a mistake, you had to start earlier in the piece, forcing you to replay sections that were good, thus perhaps introducing a mistake in those section). Suffice it to say, that punch-ins and redoing takes, and trying to get rid of mistakes on analog tape is much, much more difficult and time-consuming than with a MIDI sequencer. And wasting time correcting mistakes means that you have less time to do more productive things.
And don't forget that with analog tape, you can't slow it down, record your part at a slower tempo (so that it's easier to play), and then speed the tape up for playback. If you do that, it changes the quality of the sound (ie, changes the overtones, vibrato speed, tremulo speed, etc). You end up with a "Mickey Mouse" effect.
Oh, and if you need to change the key (ie, transpose the parts) after you recorded all of the parts, because the singer has trouble hitting the "high notes"? Too bad. Start all over again from the beginning.
Let me tell you, the first thing I did when I switched to using MIDI to play my arrangements was throw away my analog, multi-track reel-to-reel tape recorder. Good riddance!
MIDI/computers allow for more efficient recording sessions by streamlining/improving the process of correcting mistakes in the recording, and streamlining the recording process itself. MIDI/computers also eliminate the need to have an engineer with special skills, as the computer takes over some of those functions, such as "punching in" overdubs. MIDI/computers also offer efficient ways to make useful changes, such as transposing key.
And if you wanted a really professional sounding recording of your music, you had to either buy really expensive recording equipment, or you had to rent time in expensive recording studios. Remember that back in the old days, we didn't have ICs with really dense circuitry. Manufacturers had to use a lot more electronics to deliver less. I bought a Prophet 5 synthesisor. It had analog, basic waveforms that could maybe produce somewhat tolerable renditions of real instruments. It cost me $5,500 for that one synth. Today, you can get better sound than that synth out of a $100 computer audio card. I'm not kidding you. I was there. Trying to get a good sax sound out of that synth, for example, took days of programming it (ie, some of the early synths didn't even have programmable, battery-backed memory so you lost all of your settings every time you turned it off), and then you'd get something that wasn't even nearly as realistic sounding as what you can get in a cheap (couple of hundred dollars), modern instrument with digital sampling (ie, the technology used on today's computer sound cards).
And the price of going into a recording studio? We're talking hundreds of dollars an hour. You could go broke before the microphones were even setup in front of all of the instruments. You want to know how they got the drum sounds that you hear out of your basic sound card? They had to individually tune each drumhead, and dampen its overtones (toilet paper taped to the drum head in certain spots worked great -- I was a recording engineer years ago, in case you're wondering). It typically took a minimum of 2 hours to tune an entire kit, just to get the sounds that you can get out of a digital sampler today within seconds of turning it on. And you had to pay for that 2 hours of time. And don't forget the musicians' salaries who are playing for your session.
MIDI/computers allow for professional recordings to be made on a budget, in small, "home" environments that don't require things like special soundproofing. Furthermore, the technology in computer audio often eliminates expensive, time-consuming tasks, such as tuning drumkits and placing microphones/cables. The storage capacity of computers also makes recalling a given setup very efficient and inexpensive, which is important since often a setup needs to be recalled during multiple sessions.
Now contrast this with MIDI (ie, computer sequencers using MIDI, and computer sound cards with the ability to play numerous musical parts with realistic-sounding instruments). With MIDI, I can play all of the musical parts myself. (Well, really the MIDI sequencer is my "band". It obeys all of my instructions, whenever I want, even when I get a great musical idea at 3AM. Try assembling a band at 3AM in order to check out a musical idea. Good luck. With MIDI, I plug in my headphones, and start arranging my great idea immediately, and my sleeping neighbors don't even know that I'm arranging and listening to the playback of my musical piece written for a 40 piece orchestra).
With a MIDI sequencer, you press the rewind button and it's instantaneously ready for playback. No waiting for rewinding tape. You can have the sequencer automatically punch-in for you. (ie, You don't need another guy to punch the record button while you're busy playing your overdub). You can fix your mistakes just by quickly editing the individual "musical events". Most sequencers can display the data in the representation of a musical manuscript upon the computer's display. You just click on the graphical notes with the computer "mouse" to edit them. There's no equivalent to that in the old analog days.
And you can slow down the tempo to record your part. Speed it up for playback, and no Mickey Mouse effect.
And transposing? The computer does it for me. I just tell it how many half steps up or down, or tell it what new key signature to use.
And it's helped improve my arranging immensely. For one thing, I get instantaneous feedback on my arrangements while I'm arranging, just by pressing the play button on the sequencer. I don't have to wait for musicians to play my parts. (And I don't have to pay them either because "them" is my multi-timbral MIDI module playing all of their parts on all of their instruments. For the price of hiring musicians for one recording session, I've purchased equipment that I can use for hundreds of sessions). And I can play instruments myself that I never could play before. I can play the violin. I can play the flute. I can play the trumpet. I can play the timpani. I can play an elkherders' moohoo horn. How? Because I've got a keyboard that has realistic-sounding renditions of all of those instruments. (Well, not the moohoo horn. It's hard to find a good moohoo horn sample). And, I can play that. It really helps to get a feel for how to use an instrument in an arrangement when you actually get to "play" it yourself, listening to how it sounds in various ranges (ie, good samplers use multiple samples to adequately reproduce the character of the instrument across its note range) and hearing how it blends in with other instruments in an arrangement.
I'm finally getting to hear some of those old arrangements, because now I've stopped waiting for Rich or Mike to play my guitar solo (which both said that he would do years ago but never got around to learning and recording the part. Damn those useless guitar players! I say, we keyboard players should strive to make them obsolete as we did with drummers, violinists, etc. After all, it's musical darwinism, so it can't be stopped. The weaker string strummers and skin beaters were destined to be replaced by the stronger ivory ticklers). I'm playing it myself using MIDI and a good sampler with a guitar sound. And my MIDI module never complains about what I'm forcing it to play. It never says "Hey, I don't like this esoteric, weird-sounding music you write, and I don't want to play it". MIDI has been a boon to musicians who write esoteric music. Check out Frank Zappa's Civilization Phase III, done entirely on a synclavier (ie, really fancy digital sampler with built-in sequencer). I doubt you could find many musicians who could play that music, and probably none who could duplicate those highly-electronically-processed sounds with traditional instruments (even though he is manipulating samples of those traditional instruments).
With MIDI, I make recordings that sound like they were done in an expensive professional recording studio, and I do it in my home at a fraction of the cost and time and physical space. I wish I had it all years ago.
By the way, did I mention that I used to have to walk 5 miles to school everyday through 10 foot high ridges of freezing snow (when I was a four foot tall kid)? You young "MIDI kids" of today have it easy... (Hey, I had to listen to that crap from adults when I was a kid. Now it's your turn).