If you want to read more about what is a sitemap and why they matter, you can read our in-depth guide: How to Create an XML Sitemap: Tips & Best Practices.
Without a sitemap, you will struggle to get all your pages indexed. A sitemap doesn’t automatically guarantee you will be indexed and appear in search results. The sitemap is a way to show Google the pages that do matter on your site and say ‘this page exists and you can consider it for indexing’. XML sitemaps are an essential and important tool to help crawlers (Google) navigate and find new pages on your site.