MIDI

A long time ago, analogue synthesizers got invented, and they were probably mostly intended as single instruments, much like a piano or organ. Like a player-piano, people eventually invented ways to automate synthesizers so that they a user could sequence a set of notes into the synthesizer and have it play it back to them. While that synthesizer played one part, the user would go to a second synth and play the accompaniment. Artists being what they are, two synths were hardly enough, and performers quickly started one-person bands that consisted excluisvely of synthesizers.

There were two serious issues: the synthesizers had to keep time, so internal clocks were installed. But people found that these clocks had a tendency to drift out of sync with one another, so that even if you managed to start all of your synths at the right foot-tap, by the end of a six minute song, any automated sequence would be out of sync with any other given sequence.

The other problem was that there were purely physical logistical problems with one person trying to play and trigger eight different synths to do different parts over the course of a song. It just wasn't practical.