A good XML sitemap is a roadmap to all important pages of a website. This roadmap guides Google to all main content on a website. Having an XML sitemap can be beneficial for SEO, as Google can retrieve essential pages of a website very fast, even if the internal linking of a site isn’t flawless. …Read: "What is an XML sitemap and why should you have one?"
In short, XML sitemaps are a list of URLs of a website. Apart from the URLs, they can provide additional data about a page, like when it was last updated, how often it’s get updated, if a page contains images etc.
XML sitemaps help search engines to crawl your website and find all important pages of your site. Usually crawlers simply follow links on your site to find all content. Implying that if your internal linking structure isn’t flawless, they might miss out on some content. This issue can be prevented with an XML sitemap. If you have an XML sitemap that lists all important URLs of your site, you ensure that crawlers will encounter these URLs.