If your site was built correctly, i.e., it's crawler-friendly, you certainly don't need a Google Sitemap. It won't hurt you to have one, and you may be interested in Google's other Webmaster Central Tools, but having a Google Sitemap isn't going to get you ranked better....
Sitemaps are awesome for idiots that can't find something on a website. I find myself using sitemaps on website often so it really helps me out when Im stuck. Then theres the benefit of using it so spiders can follow the architecture of your website easier. IF you put a link to the sitemap in your footer, then your telling bots that your sitemap page is important, thus distributing weight throughout your pages more evenly. I'm all for sitemaps.... its not necessary but I think they help out a lot for both end users and bots.