Busses have mainly been kept in Logic 8 for compatibility purposes with Logic 7 projects...
In Logic 8 a Bus is like a virtual cable that takes an amount of a signal "sent" from a channel to an Auxiliary track.
If you send say 0db from a channel via Bus 1. Then Logic creates an Aux track. That Aux track will have the Input "Bus 1".
An Aux, can then have many purposes.
Say you have 10 tracks for your drums in the Mixer/Arrange. Sometime after you've mixed them relative to each other you may want to 1. change the level of all of the parts and
2. add an effect to all 10 drum parts.
Well, for 1. yes you can group or send them to an Aux to do this. But for 2. adding say a reverb plug-in to all 10 channels will be very cpu-heavy. Better to send an amount of all 10 channels to an Aux channel and place one reverb unit on this Aux!
Another benefit of sending an amount of the signal from a track(s) to an Aux for reverb or any other effect, is you can easily change the wet/dry amount.
If you create an mix your tracks in stems (drums mixed and bounced together, strings mixed and bounced together, etc.) then using Aux's as mini-master level channels is a really convenient way to organize and ultimately control, automate, mix relative to each other and bounce...
I'm sure there are other uses too that I haven't covered.
Does this help?