Google search engine has implemented a Beta version of their sitemap service in June, 2005. This allows the website owner to create a sitemap using XML and submit it to Google. When the Google spider crawls the site, the information provided by the sitemap will be used. With this sitemap, you can specify priority, last modified, and frequency of modification. This will help Google figure out how often to re-crawl a page for changes and its importance. This does not replace their old method of searching and indexing pages, but rather supplements.
At this time, the feature is new and the advantages and disadvantages of implementing a sitemap have yet to be determined. However, since Google is offering a new feature, it is in their best interest to make it an advantage for website owners in order to have this new initiative take off.