An SEOMoz post has documented statistics on how submitting a Sitemap to Google and Yahoo impacts the rate of crawling that web site. The results show that submitting a Sitemap to Google and Yahoo decreased the time it took Google and Yahoo to crawl the page.
It is important to note that the test was done “on 12 different posts, 6 with Sitemaps being submitted, and 6 with the Sitemaps not being submitted.” The results were that when a “Sitemap was submitted, the average time it took for the bot to visit the new post was 14 minutes for Google and 245 minutes for Yahoo.” In contrast, when “no Sitemap was submitted and the bot had to crawl to the post, it took 1375 minutes for Google and 1773 for Yahoo.” That is a huge difference in crawl time. It is true that many sites see their content indexed within minutes, with or without a sitemap file. But in some cases, it takes longer.
In the old days, I was not a fan of submitting sitemap files for sites that were very search engine friendly. But sometime last year, I became a fan of using sitemap files for search engines. Mainly because it gives me an extra metric in the webmaster tools area to glean information. But in addition, I have been hearing positive results from webmasters after using it over the course of 2008. Finally, I have been noticing that Google is pouring more and more time into these tools and using the data to improve the search results – so why not be there in most cases?