An XML sitemap is a file (sitemap.xml) listing all important pages on your website with metadata like last modification date, change frequency, and priority. You need one to help search engines discover all your pages efficiently, ensure new content gets indexed quickly, provide metadata about page importance and update frequency, and improve crawl efficiency for large or complex sites. XML sitemaps are especially important for new sites (limited external links), large sites (thousands of pages), sites with poor internal linking, and sites with frequently updated content. While not mandatory, sitemaps significantly improve indexing coverage.