What is a sitemap?
A sitemap is a exactly what it says, a map of pages you want the search engines (and people) to know about. Generally they are for search engines to quickly read and index your site.
A sitemap is more important for a new website or for a site with poorly designed navigation. But it is widely held in the SEO community that older established sites do not benefit from a sitemap. Having said that, every website should have a “viable” sitemap of some sort. Some major website vendors may not include a sitemap.xml file. Some vendors produce formats that are more user-friendly in .aspx or HTML, but users seldom need to access a sitemap (on a well designed site).
Google recognizes several sitemap formats just fine, but XML is still the only way I would suggest to guarantee your site is properly indexed by a wider range of search engines.
If you have a sitemap, the sitemap file must be updated every time a change is made to the site. Having a static sitemap is far worse than not having one at all. Check yours to see that it is updated (updating automatically). Some vendors update sitemaps manually. That is a bit on the crazy side because there are plenty of scripts out there that will automate the process.