Google Webmaster Tools provides a ton of useful data for Webmasters. In providing an XML Sitemap and submitting it to a Google Webmaster Tools Account a Webmaster can tell Google to crawl the pages included in the sitemap. This is useful in two ways. First it sends Google to the site before it would find it's own way there effectively having content indexed in Googles Search Results sooner. Secondly, it allows Google to review the site from the view of a Search engine, the providing useful information to the Webmaster about the site.