Imagine you have a document consisting of a number of pages that contain text and images. Now you need to change the page size to be smaller or larger.
One way to do this is to choose File > Document Setup, change the page dimensions, and click OK. While this works, it gives you no control over where the extra page width or height is added or removed. The rule is that the page enlargement or reduction is performed equally on all four sides, except in the case of facing pages documents where the width change happens away from the spine.
But, in CS5, you can control this behavior. Instead of using Document Setup, select the new Page Tool in the Tool panel (third tool from the top). Next, select a "reference point" on the proxy icon in the left end of the Control panel. Then adjust the width and height fields in the Control panel. The reference point you've chosen will remain fixed, and the page will grow or shrink in relation to it.
I've found this very useful when creating tablet screens for the Adobe Digital Publishing Suite. One option with the Digital Publishing Suite is to create pages that are longer or wider than the tablet screen, allowing the user to swipe up and down or left and right to view a single long (or wide) "page". It can be really handy to create a 1024px x 3000px long page, add the page content starting at the top, and then just shorten the page from the bottom when finished. The technique above makes this painless!