I use it for generating music through physical models. If the model requires a very numeric-sensitive algorithm like a differential equation solver, it's nice to have the extra precision, and usually performance doesn't matter too much in these sort of things (because ultimately, I just click "render" and wait anyway). To be very honest, I don't even know if I need 128bits, maybe a 32bit float would do... I never ran a comparative analysis. It just gives me more room for error, so that I can focus on the music/sound and not too much on the numerical analysis/programming. Working on hobby music+programming projects is the ultimate freedom.