Kiki would be better at answering this than myself, but I'll give it a whirl.
The buttplug is used for many different concepts; dominating, anal training, the feeling of being in public and no one knows but you, etc.
Using a butt plug is neat because you start out small and increase the size slowly until you get to a size that you are comfortable with, ex. partners penis. This apparatus comes in a zillion of different sizes too.
I'm in class and feel uncomfortable doing this right now. Maybe I'll get into more detail when I get home.