A sitemap is essentially a list that tells the web crawlers (bots) which pages on your forum that you think are important. Sitemaps are created in order to help the bots crawl your website more intelligently, with a direction. The sitemap also provides the bot with other information, such as when the page was last updated.
When no sitemap is present, search engines will simply crawl by visiting links it finds on your site.
Sitemaps actually aren’t necessary for forums since they have a very clear tree-like content structure, which makes it very easy for bots to navigate. If, however, you do want to incorporate a sitemap, Vanilla offers this as a plugin feature.