Google likes to crawl a site by following the internal links. It can then build a full map of the site and from there determine which pages are important (eg categories) and those that don't necessarily need to rank (eg privacy and legal pages).
Google makes it very clear on their sitemap guides that a well built site doesn't need a sitemap. Google does say most sites will benefit from a sitemap but neglects to say a poorly created sitemap can harm your ranking.
If your site has already been crawled subsequently submitting a sitemap is pointless.
If you have a fast changing ecommerce site you will be submitting sitemaps daily making the whole process pointless (Google doesn't crawl that often).