If Google can go to a site and see the entire site using the navigation on the page, then in many cases, a site map wouldn't provide much help. If you have pages that are deep linked, then that's where a sitemap comes in handy. We're taking sites with 50+ pages, ecommerce sites and active blogs etc.
The sitemap just makes it easier for Google (and Bing) to understand where the pages are located. They'll still crawl your site and discover things on their own, but the sitemap is providing them a list of "here, I want you pat attention and look at these pages".
With all that said, if you're using WordPress, Joomla, or other systems, there are plugins for generating a sitemap automatically, and free, so set it up and submit it - it doesn't hurt. Larger sites, it's required, but I add it to every site we build, even a site with only 3 pages
