Backdrop is a great little utility for MacOSX which allows you to cover up parts of your screen while you take screenshots, or am demoing something without having to clean up the rest of your windows. Basically, think of it as an in-between layer.
Or how’s this for an analogy. You don’t wanna clean up your room, but your mom insists since company is coming over. Wouldn’t it be nice to have some “in-between layer” that you could throw in while company was there, but not mess up your “style”? There you go.