Are you speaking relative to Sonar here?
In Sonar, the aux busses are typically used to apply the same FX to multiple tracks to reduce CPU consumption. For ex., if you have a mix with 3 bg vox, rather than slapping a reverb on each track, you can route them to an aux bus and use just one reverb instance on the bus for all 3 tracks.
Busses on a h/w mixer are frequently used to create submixes. For ex., you can send a drum kit to a bus and use the bus fader to raise or lower the volume of the entire kit, while still using the track faders for adjustments within the kit (louder snare, etc.).