Site Maps for SEO
You might have noticed that a lot of sites you visit, even some of the smaller ones, have a little link somewhere, maybe towards the top right or bottom right, that says "Site Map." On larger sites, this is very useful for us humans that tend to go through sites by "browsing" (as opposed to searching or taking shots in the dark by clicking on navigation elements).
Site maps are actually pretty useful to Search Engine spiders as well, because most of the automated "robots" that scour the Web looking for new content actually behave like humans that are browsing a site, clicking various links on a page. Having one page that is linked right off the home page with a complete directory of a Web site makes it much easier for these robots to properly index your entire site.
It's also helpful if you have a large number of dynamically served pages that might be hard to get to otherwise.

