David explains how a site map benefits search engines and your website. You can get search engines crawl more pages of your website, and gathering more keywords and content. The gathering of this content allows search engines to display your results higher up, and to more relevant searches.
Site Maps are pages that contain links to all the pages of your website, or to the most important ones, so search engines can easily find them. You can think of a site map as a table of contents for your website.