As I'm sure most people will be aware a sitemap is one way of letting Google know more about the structure of the pages and links within your site.
This facilitates the smooth transition of Googlebot from one page to the next and speeds up indexing. I've used sitemaps for quite a while and I'm struggling to see any discernible differences in the index/crawl rates of sites with and without sitemaps.
Using sitemaps has many benefits, not only easier navigation and better visibility by search engines. Sitemaps offer the opportunity to inform search engines immediately about any changes on your site.