Sitemaps help search engines discover pages, especially on new sites with few backlinks. A single sitemap can hold up to 50,000 URLs; larger sites use a sitemap index.
Being in a sitemap doesn't guarantee indexing. Pages still need to be useful and reachable through links.
Why it matters when you launch
Submit your sitemap in Google Search Console and Bing Webmaster Tools on launch day so your new pages get crawled quickly.