There are many advantages of a site map as it is useful for your visitors in navigating and finding their related information in a quick and effective fashion. Google Sitemap serves the same function for spiders and bots.
There is a post at SEOchat which lists the number of ways in which the Sitemap can be useful. With Google Sitemaps, bots and spiders can easily find and index your site. Google Sitmap is written in a language that is interpreted by automated trawlers that cover the web for getting the URLs. The language is the Google created Sitemap Protocol. Since Yahoo! and MSN have agreed to support the protocol you can submit Google Sitemap to other search engines too. With it you can tell Google about your website and URLs it has. With Google Sitemaps you can have the information such as the when the pages were updated. This will save the time and bandwidth as the crawler will know that if there has been no update of the page from the last indexing than there is no need for indexing again.Many Advantages Of Google Sitemap ,