Many people think that Google sitemaps are worthless, but let me tell you the .
Google Sitemaps is one of the biggest inventions made by the Google. Recently, Yahoo has also taken interest in it and even the “Yahoo SHLURP” bot is requesting the sitemaps.xml file.
The sitemaps is a map of your well-structured website which contains each individual url and its priority for the website.
The priority is very nice to be set up carefully and therefore put larger value to the main page www.example.com and less value on the pages that are not going to change after their creation.
Because the higher this value is, the highest priority in their list this url will have and will be the first indexed on a deep crawl…
On the other hand the Update rate value is really worthless thing because most search engine do not obey it and that‘s a well known fact.
Search engines will visit your url only if they stumble across a link to that page and in most cases they will index only the page that is linked.
During the deep crawl (which is done once a month) then the whole page with all the urls will be indexed.
Don‘t think that when Google opens your website it will get the whole thing at once .. nope .. the situation is a bit different and Google will index few urls, then make a pause and come back again until the whole website is indexed.
One more benefit of the sitemap is that you can add it on the Google webmasters Center to verify it and to get the highest benefits out of it.
Suppose if you’re using the WordPress it will automatically ping or send a request Google to get the sitemaps file.