It works like this. You go to a computer music show, and you see a keyboard hooked up to a program.
Someone plays something on the keyboard and as they do so, notes appear on the computer screen.
It looks wonderful. Sometimes the notes disappear as the next part of the demo follows on so quickly that you don't really get time to see if they are good. Sometimes you do get a chance to see, and it is really good. But the player was a professional musician. The first bar is just fine, but then you hesitated for a split second and all the following notes are shifted and split across bars, and some of the notes aren't quite the length that you meant.