there are a few tutorials that show you creative and practical ways to use buses/aux's.
For example the Logic 303: TNT is great and shows in depth use of buses in the use of side chaining.
The new Logic 404: also delves into using buses as a way of grouping parts (for example hihats, snare, toms, kick drums to a Bus that enables an easy way to mix in stems).
Section 24 (chaps 177-182) in Logic 101: Core Logic 8 also looks at the uses of buses/auxes.
If I have 2+ tracks that will need the same effect or reverb I will often place the reverb plugin on a bus and route the desired audio signal from some tracks to that bus. The benefits being
1. I can choose how signal from each track passes through the bus (and therefore is affected by the reverb) and
2. It's easier on the computer to run one reverb plugin than 2 or 3 or however many tracks you want to have reverb on.
There are many ways to use buses for cpu saving reasons and creative reasons...
The kind of music you make might help you figure out the way that you use buses - as will your individual workflow that you develop.