The crossover is a cutoff point. Everything under the chosen crossover will be sent to the subwoofer, instead of to the specified channel. You would want to use this so that you don't "lose" chunks of sound, and to get the most out of your subwoofer.

For example (assuming you can set independent crossovers for different channels):
If you have qs8, they are rated from 95-22K. If you set your crossover for your surrounds to be 80Hz, all the sound above 80Hz will go to the qs8s, and below will go to the subwoofer. This leaves a band of sound from 80-95 Hz that might be "lost" (depending on how steeply the qs8s drop off below 95Hz). For this reason you might want to set your crossover for your surrounds to be 100Hz.

That said, a lot of people just set the crossover at 80Hz for all channels and leave it there.

[black]-"The further we go and older we grow, the more we know, the less we show."[/black]