From Yamaha Wiki
The Yamaha MT-100 4 Track Recorder was an Analog circuit tool developed for midrange recording hopefuls in the late 1980's. It was marketed just before the advent of Digital Audio Tape. In essence it allowed variable speed recording of 4 tracks of Sound that could be mixed and merged and re-recorded. It had its broadest applications with keyboard and guitar players. It would not have been extremely useful for drummers who require about 8 channels for an entire kit.
What makes the Yamaha MT-100 so appealing to people working with Samplers and Synthesizers of the era is this single selling point. Many of them had a multi-track Music sequencer built in. With the combination of 8 to 16 tracks, one was able to Multiply the number effectively up to about 128 tracks. 256 with creative mixing.
These were also brought to being because computer gear at the time was expensive and hard to come by for many aspiring youths of the era. But they were inspired by many publications like Keyboard Magazine and Guitar Magazine in the US and abroad to use their talents creatively.