The site map is a listing of all of your sites templates, and all pages created from each template. It is a good resource to get to pages that you can't find through navigating your web site, or to undelete pages that you may have deleted earlier. Also useful to determine what pages are made from what templates.
It is also a handy resource to see what pages link to other pages. For example you can click on 'linked pages' next to your about page to see what other pages link to your about page. Handy if you are about to disable the page and need to identify links first.