I guess the theory is that, even though we may not be able to hear it, there may be some inaudible charateristics that affect the way that we interpret/recieve the sound. Or, it may be that, even though we can't hear above 22kHz, a speaker that can play frequencies above that amount is sure to be able to reproduce the frequencies we CAN hear without strain, and as such, the extreme frequencies are experienced as naturally as possible.

But what do I know.