What a sitemap does for a site nobody links to yet
Google finds pages by following links. If your site is new, or a page sits four clicks deep with no internal links pointing at it, discovery can take weeks. An XML sitemap is a direct list handed to the crawler: here is every URL I consider worth indexing, and here's when it last changed.
It doesn't force indexing. Google decides what's worth keeping based on quality and demand. What a sitemap reliably does is shorten discovery time and give you a clean diagnostic in Search Console — submitted versus indexed is the single fastest way to see how much of your site Google actually wants.