I also use them to anchor people, both to get around problems with poses that move the character to 0 (since if they are hooked to a null that 0 is relative to the null, hence they don't move), or when trying to pose two or more characters in relation to each other - you can move the null without changing the relative positions of the characters to each other. Just be careful not to load too much on to the same null when you do that or it will slow your machine to a crawl, particularly if smoothing modifiers are active for clothing.