The sitemap not only helps for indexation and site discovery it also helps to de-index obsolete pages more quickly. In case a page no longer exists on the site (returning a 404) and in parallel also no longer is mentioned in the sitemap. Google will de-index it more quickly. I.e. I will re-crawl the page less often before it is convinced it is actually gone.
So it appears Google keeps “a state” of the sitemap and looks at differences.
That 2nd use of the sitemap makes it a powerful tool in combating index size issues and ensuring a domain’s “crawl capacity” is used on pages that the webmaster wants to get crawled.
HTML site map is for visitors and help them in navigating through the website (which Google likes). So both of them have importance in their own ways and helps your website in SERP ranking. Make sure your site maps include all pages and their sub pages. Make the site map like a index page of a book. It needs to be clear and logical.
