Well, for one, some users have gotten used to using a sitemap instead of stumbling through a site's navigation.
Equally, or more, important, a sitemap is a guarantee that search engines will find the pages you want them to find. Maybe your site has flash linkage to a specific landing page, or one of your URLs is broken - an updated sitemap will be read by the search engines (and thus, indexed).
Finally, we like to research keywords and planned seo strategies using the sitemap before getting started with the website design. This is one of the best ways to ensure a successful design leads to a solid build and happy clients, visitors to the site and search engines!
Oh, did we mention you can also use the sitemap to include meta data about the types of content on your site (video, images, software, mobile content, etc).