The Yamaha DX7 is a keyboard that uses the FM synthesis method of tone generation. It is over twenty years since it was first released, but it still sounds pretty good today – if you use it for the sounds which it is good at. The DX7's electric piano sound is very satisfying to play, almost as satisfying as an acoustic piano. Although some keyboards are more convincing, they don't give player-satisfaction in the same way.
The most significant feature of the DX7 however was its role in the downfall of the Musicians' Union. The MU was once an all-powerful trade union. Go against the MU and suddenly you didn't have any musicians. The BBC found this out in 1980 when they tried to disband some of their orchestras. The union went on strike for nine weeks, during which time the entire BBC had no live music.
This period marked the peak in union membership. It was a time when theater producers in London's West End had to go to the Union and ask them how many musicians they needed to employ. Not how many the production needed, but how many the Union said were necessary. And of course to go against the Union was to get no musicians at all!
The thing about the Yamaha DX7 though is that it was the first keyboard that could produce a sufficient range of sounds, with 16-note polyphony when the standard then was far fewer, to produce most of the range of sound timbres of an entire orchestra.
So a theater pit didn't need an orchestra, it needed a small band including two or three keyboard players. Granted, by the time this became commonplace, the DX7 wasn't the only keyboard in use, but it was the first that had this capability.
The Musicians' Union held out for a while against the tide, but eventually they had to relent. Having said that, there is no doubt that the sound of a big orchestra with conventional instruments is better than the keyboard imitation. But economic practicalities make this impossible these days.
Oh by the way, the Musicians' Union is still very much in business. They help musicians, but the old days of restrictive practices have very much gone, thanks to the Yamaha DX7.