When you walk into a shopping mall there is usually a map or directory pin pointing the locations of all the stores. Site maps are very similar in nature. The site map directs the search engine spiders to all of the relevant URLs on the site. This allows the subpages and landing pages to be easily accessible to the search engines for quick indexing. This helps ensure that all pages are indexed properly and none are left behind.
Often times, we pay close attention to what we keep from Google using robot.txt files. Site maps work just the opposite. This is the content you are sending directly to the search engines.
SEO site maps are different than the HTML site maps designed for human visitors to see. These are written expressly for the search engines to ensure full indexation of your site. XML site maps give you the chance to let them know what has changed on your website. This is not a guarantee that Google will rush out and index the pages that change, but it is a leg up in the process.
An optimized site map is an important part of your overall search engine optimization strategy.