What an XML sitemap is for
An XML sitemap is a file that lists the URLs you want search engines to crawl and index. It's a discovery aid, not an index command — submitting a URL doesn't guarantee it ranks, but it does make sure Google knows the page exists.
Sitemaps matter most where internal linking is thin: brand-new sites with little authority, large sites with deep pages, and pages that aren't linked from the main navigation. If a page has no path in from your other content, the sitemap may be the only way a crawler finds it. For a small, well-linked site, the benefit is smaller — but a clean sitemap never hurts.