Xml SiteMap is simply a page that lists all the urls with meta data likes when they are updated the last time, how often is it updated and other data likes its priority (how important is it). Though Search engine crawlers like Google bot are intelligent enough, they may not crawl all the urls of your site. For example if your site uses Java scripts, Superfish menu Or any kind of flash menus, then the google bot can't index your sute properly. Here comes the importance of a sitemap.xml file. Since the sitemap file stores all the url, no urls of your site will be disappeared from the search results.