Google SiteMap and the other web tools provides a way for the owner or webmaster of a site to easily submit all of the pages to Google and keep track of which ones are crawled and indexed. Wikipedia provides a much more detailed description which goes beyond the scope of this post.
I didn’t want to have to manually try and figure out which links are children of other links for my own site (which again only has 5 pages anyway!) or other things like that, so I was looking for an automatic way to generate the google sitemap and then be able to provide it to google.
MODx Revolution has a plugin called GoogleSiteMap which uses a snippet called GoogleSiteMap I created a page, set the template to empty, and called the snippet directly on the page and then took the xml it created and pasted it into an xml file to submit. The XML it generates even correctly does the last modified date.
Hope this helps!