Google introduced Google Sitemaps so web developers can publish lists of links from across their sites. The basic premise is that some sites have a large number of dynamic pages that are only available through the use of forms and user entries. The sitemap files can then be used to indicate to a web crawler how such pages can be found. Google, Live and Yahoo now jointly support the Sitemaps protocol.
For example, think of a forum that is using dynamic pages. Google would only return less than 100 results without using a sitemap. Once a sitemap is provided, there are more than one millions pages in the result. Since MSN, Yahoo, and Google use the same protocol, having a sitemap lets the three biggest search engines have the updated page information