What is HTML Sitemap?
An HTML sitemap allows site visitors to easily navigate a website. It is a bulleted outline text version of the site navigation. ... This Sitemap can also be created in XML format and submitted to search engines so they can crawl the website in a more effective manner.
A sitemap is (usually) an XML document, containing a list of pages on your website that you have chosen to tell Google and other search engines to index.
HTML sitemap is a plain text sitemap used by browsers to know and surf through your website pages.
.html sitemap is for better navigation of the website specially for the visitors and for the search engines to notify them that you have these existing pages updated.