You can create background boxes with it to make your UI look cleaner (or more cluttered!) and move your unit frames, chat boxes and other things over it. Basically, it's a UI enhancement feature that does nothing but create nifty boxes that you can do basically anything with if you know how to use it.
Meh...the complexity to usefulness ratio doesn't seem to add up for me, I am sure it can do nifty things, but it doesn't seem to nifty to the point where its a must have for me =(.