A sitemap is a file where you provide information about the pages, videos, and other files on your site, and the relationships between them. Search engines like Google read this file to crawl your site more efficiently. A sitemap tells Google which pages and files you think are important in your site, and also provides valuable information about these files. For example, when the page was last updated and any alternate language versions of the page.
If your site's pages are properly linked, Google can usually discover most of your site. Proper linking means that all pages that you deem important can be reached through some form of navigation, be that your site's menu or links that you placed on pages. Even so, a sitemap can improve the crawling of larger or more complex sites, or more specialized files.
You might need a sitemap if your site is really large. As a result, it's more likely Google web crawlers might overlook crawling some of your new or recently updated pages. You also might need one if your site has a large archive of content pages that are isolated or not well linked to each other. If your site pages don't naturally reference each other, you can list them in a sitemap to ensure that Google doesn't overlook some of your pages.
Another reason to have a sitemap is that your site is new and has few external links to it. Googlebot and other web crawlers crawl the web by following links from one page to another. As a result, Google might not discover your pages if no other sites link to them. Finally, you might need one if your site has a lot of rich media content (video, images) or is shown in Google News. If provided, Google can take additional information from sitemaps into account for search, where appropriate.
You might not need a sitemap if your site is "small". By small, we mean about 500 pages or fewer on your site. (Only pages that you think need to be in search results count toward this total. or your site is comprehensively linked internally. This means that Google can find all the important pages on your site by following links starting from the homepage. Other reasons include you don't have many media files (video, image) or news pages that you want to show in search results. Sitemaps can help Google find and understand video and image files, or news articles, on your site. If you don't need these results to appear in image, video, or news results, you might not need a sitemap.