XML sitemaps help the search engine find deep links to your website. If you have links to every page on your
site (internal or external links) then you don't need a xml sitemap, but its good practise to add one. You can also
use it to tell the search engines how often to visit per page. If your site is big like a forum, I would say
its essential to get a sitemap. There are plenty of mods you can get. If you use wordpress, they have a xml sitemap generator.