Sitemaps are an absolute necessity for SEO. It contains the complete list of URLs on a website Google needs to crawl and index.
There are two types of sitemaps, XML, and HTML.
An HTML sitemap lists all the pages on a website and enhances the user experience. HTML sitemap is mainly for users but helps in Google as well to an extent.
XML sitemaps are purely for search engines. An XML sitemap helps webmasters to let Google and other search engine bots know specific URLs to be crawled. Additionally, one can provide priority and update frequency of the content.