When you use absolute positioning, it takes the element outside of the normal flow of the document. This can come in pretty handy. If you a box, 250 pixels by 250 pixels, and you need to attach an image in the lower right corner – regardless of how much content is in the box there are a couple of different ways you can try and go about it, but the easiest, by-far, would be to just set the positioning of the parent container (the box) to relative, and then give the image a position of relative, bottom: 0, and right: 0.
Look at this page, see the elements attached to the bottom of the page. That would be pretty difficult to accomplish trying to use floats and margins.