Google have updated Sitemaps in order, I imageine, to make it more attractive to webmasters. You can now, whether or not you have a sitemap, view statistics about what search terms your site is ranked highly on and what you get clicks from. All you have to do is to add an empty file with a certain name to your webserver to prove you have access. If you want you can also set up a proper XML sitemap, there is a WordPress plugin to do this which I am now using.
It also gives you statistics on PageRank, robots.txt, common terms used in links to you and common words used on your website. I strongly recommend at least viewing the statistics!
[Via Google Blogoscoped]