Thanks guys, lots of enlightening just went down over here. so, say I wanted to record a piano part, but I didn't know what patch to use, ect ect, this is something I can use midi to mess around with, without having to re-record the piano part. Also, By drum machine, I mean this Electric drum pad I have that "uses" midi. Thanks again.
Yup! Record just the midi on as many instruments as possible (unless you have great instruments and great mics, then keep the audio tracks as well). Then if you don't like the sound of the piano, or you think you should have used a Fender Rhoades instead of a Steinway Grand, you can change the sound to a Rhoades without having the keyboard player come back to the studio. Record the midi drum hits that the drummer plays, then use EZ Drummer or Superior Drummer or some other DAW based drum collection to decide what kits and what drums you want to use for the audio sound of the drums. If some of the timing is not perfect, use the mouse to move the midi notes to correct it. Or correct piano mistakes by using the mouse to change wrong piano notes to the correct key numbers.
If you still have an outboard rack full of black box effects, midi can control all of your fx (assuming the fx boxes are made to receive and comprehend midi commands, most newer ones are, but even older ones will at least understand patch change commands), and it can automate your mix as well so you don't have to ride faders when you mix down for the final.
Several years ago, I had an Atari 1040 and a Tascam 2488 and a 7 foot rack. The 2488 was slaved to MTC (Midi Time Code) so it would start and stop under control of the Atari. All things midi were recorded (midi only, not audio) and played back by the Atari, which would select all the correct synth patches and fx patches for me, with the synth and fx audio routed to the 24 channel Mackie mixer. (The Atari was essentially "playing" the synth instruments "live" every time I started the song, since the audio coming from the synths was not recorded! ) All the audio (non-midi) sources were recorded on the 2488, and all of the 2488 faders and pan and fx settings were also controlled by the Atari by using midi. The only thing not controlled by midi was the Mackie mixer, simply because it was too old to have midi control, and I could not afford a midi controllable mixer.
When I was all done wtih a song, I could literally hit the spacebar on the Atari, and I could watch all the synths and fx boxes blink with patch changes, watch the 2488 synch and start in time with the midi tracks on the Atari, literally watch the Atari run the entire show in real time and sit back and listen to the finished mix. As stated above, I could have had lights and fog machines controlled by the Atari also......
Indeed, what can you not do with midi?