Sitemap automatically creates a dynamic XML sitemap of your entire site and informs search engines exactly which pages on your site are available for crawling.
Sitemap allows you to choose which sections, category groups and Commerce product types to include in your sitemap, as well as to select each a change frequency and priority for each. The purpose of this is to help search engines to more intelligently crawl your website, with the ultimate aim of improving your SEO and pagerank.
Note that this plugin create an XML sitemap specifically for search engines, and not a HTML sitemap for your front-end site. Find out more about sitemaps at sitemaps.org.