As the name implies it automatically Create a SiteMap of your web page. People love it and It also can do a lot more.
For instance SiteMap can:
* Create a 'siteindex.xml' file for to help google index better.
* Validate that your web page doesn't have any nasty defaults in the title, filename or folder name (such as "Untitled Page 3" or "page4/pag36.html").
See this for one users take
* Easily turn off web crawling for a given page within your site.
* And yes, SiteMap can also create SiteMaps in a variety of styles (Including, with version 1.14 your blog entires!)