Let's see - the rubber band is so useful in binding things together that you don't want to staple or clip. It's stretchy! It's flexible! You can always add one more thing to the stack that is being held together by the little gizmo.
I use them for many everyday things but I also use them in the process of making "Diaper Cakes" for baby showers. I wouldn't think of tying to make one without them.