In CS6, I create two shapes and apply the different compound shape tool options from the property inspector.
1. with add/union it puts two shapes together, however, if I take the subselection tool I can simply move the separate shapes. How is this different from just putting one image on top of another and grouping them?
2. Can anyone give me any examples of how any of the compound shape tools might be used to create buttons or interface elements?
The Compound Shape tool makes it easier to build more complicate vector graphics from simpler shapes, and gives you the allows you to do things like punch or union wihout permanently combining multiple objects.
Below is a link to a free exceprt from my Fireworks CS5 Classroom in a Book, which disucsses the Compound Shape tool. Its functionality has not changed in CS6.
Also, attached is a VERY simple example of using Compound Shapes and regualr vectors for interface elements. There are people far more visually talented than me in this forum, but I hope this gives you an idea. This is an editable FW PNG file.