Google sitemaps are google’s way to allow a webmaster to submit all their siteURLs to the Google index and be able to view detailed reports about the visibility of their website pages on Google.
Further to this the sitemaps can be updated and keep Google aware of all the pages in the site and, most importantly, when changes are made to specified pages to help improve the efficiency of the Google visits to the site and subsequent indexing. The goal is to improve your search engine optimisation activities.
In the last few years Google has loved Blogs as they have used the ping mechanism to advise external services and indexes when an update has been made to the site. In this way Google can ensure that it visits a Blog when there is new content and not waste a visit when there is nothing new. When your search activities are carried out on the scale of Google’s wasted visits cost money.
Sitemaps help to enable a similar method of working for more traditional web sites and might lead to Google prefering sites which post sitemaps in its new service rather than ‘not cooperate’