Why Sitemaps Matter for Sites Search Engines Can't Fully Crawl
A sitemap is a direct list of URLs handed to search engines, supplementing (not replacing) their normal link-following crawl process — it matters most for large sites, sites with pages that aren't well-linked internally, or sites with content that changes frequently enough that waiting for organic re-crawling would be too slow.
Each URL entry can carry metadata beyond just the address: lastmod (when the page last changed, helping crawlers prioritize re-fetching updated content over unchanged pages), changefreq (a hint, not a guarantee, about how often a page tends to update), and priority (a relative weighting between a site's own pages — it has no effect on ranking versus other sites, only on how a crawler might prioritize within your site).
A sitemap doesn't force indexing — search engines still apply their own quality and relevance judgment to every URL — but it does ensure a page is at least discoverable, which is a prerequisite for indexing that orphaned or deeply-buried pages might otherwise miss entirely.