Google SiteMaps allows you to tell Google which web sites it should crawl and it allows you to tell Google that the content on your site has changed.

Submit your sitemap to Google Sitemaps
The easiest way to submit your web page URLs to Google Sitemaps is to create a list of URLs in plain text format:

• http://www.example.com/index.htm
   
• http://www.example.com/page1.htm
   
• http://www.example.com/page2.htm
   
• http://www.example.com/directory/whatever.htm

Enter one URL per line and save that list with the file ending .xml. Then login to Google Sitemaps and upload that file.

Does this increase your Google rankings?
No, it doesn't. Using Google Sitemaps will not influence your rankings. You just tell Google that your web pages exist. Google still uses its normal ranking algorithms to determine the ranking of your web pages.
Google even makes no guarantees about when or if your URLs will be crawled or added to the index if you use Google Sitemaps.

What does this mean to your web site?
Google Sitemaps might be useful to you if your web pages aren't crawled by Google's spider. However, you won't increase your Google rankings by using Google Sitemaps.

If Google's spider hasn't crawled your web site yet, your web site probably doesn't have enough good incoming links. If your web site has many good incoming links, Google's spider will find your web site anyway. In addition, good incoming links tell Google that your web site is important and that it deserves high rankings.