A sitemap is a file where you can list the web pages of your site to tell Google and other search engines about the organization of your site content. Search engine web crawlers like Googlebot read this file to more intelligently crawl your site.
Also, your sitemap can provide valuable metadata associated with the pages you list in that sitemap: Metadata is information about a webpage, such as when the page was last updated, how often the page is changed, and the importance of the page relative to other URLs in the site.
You can use a sitemap to provide Google with metadata about specific types of content on your pages, including video and image content. For example, you can give Google the information about video and image content:
A sitemap video entry can specify the video running time, category, and age appropriateness rating.
If your site’s pages are properly linked, our web crawlers can usually discover most of your site. Even so, a sitemap can improve the crawling of your site, particularly if your site meets one of the following criteria:
Using a sitemap doesn't guarantee that all the items in your sitemap will be crawled and indexed, as Google processes rely on complex algorithms to schedule crawling. However, in most cases, your site will benefit from having a sitemap, and you'll never be penalized for having one.