An XML sitemap is a website guide that site owners give to Google to keep track of their main content on a website. Creating and submitting an XML sitemap to Google Search Console or Bing Webmaster Tools tells them that you consider the pages in the XML sitemap to be quality landing pages that merit indexation and regular revisits. An additional value of the site map is that it allows a site owner to provide the search engines with a list to review providing a much more efficient way for them to digest your website. While Google and Bing will also crawl your website via a more natural manner going from page to page via the links on those pages, this takes a lot more time and could result in pages being missed or not rechecked often enough based on changes you make in the day-to-day of managing your site. Additionally, the XML sitemap(s) from your website should be included in your robots.txt file so that the search engines know exactly where to find them in the root directory.