Sitemaps
Sitemaps are XML files that outline website URLs for search engines to easily access. Elements of the sitemap include location of file, priority of page, frequency of updates and images included on the page (among other elements).
Sitemaps are ideally located at the root of a website (www.example.com/sitemap.xml), however, unlike robots files, they can be located anywhere on the domain.
Sitemaps are not a required element, but are highly recommended for large sites. Google and other search engines are under no obligation to crawl and index all the URLs indicated on the sitemap, but they can use them as a map to identify potential sites for crawl. Additionally, while it’s general practice to include priority and update frequency, Google may not factor in these elements anymore. There is no harm in including them, and these items should remain until further industry confirmation of the omission assertion.
Once built and hosted, sitemap files should be submitted directly to search engines. Bing/Yahoo and Google allow Webmasters to submit sitemaps directly via Webmaster Tools (Bing/Yahoo) and Search Console (Google)