Noise. Either directly as a source, which is then filtered to produce say a cymbal sound, or indirectly by creating timing variance, ie notes not playing exactly at the beat each time, which makes things sound less artificial and more pleasing.
It can also be used to dither[1] the result when doing a final rendering, to improve the noise floor.
Ah, ok. I was thinking playback. Noise makes total sense. Thanks!
I guess I should have pointed out the noise shaping could be useful if you're doing 16bit playback of a 24bit source, so possibly useful during playback as well.