An HTML sitemap is intended for visitors (unlike the XML sitemap made for search engines). Made in a form of a bulleted list, it displays links to all the pages on the website. It shows its hierarchy and helps visitors with navigation.
It can be used during the development phase to ensure a good structure that will both spread link juice efficiently and make sense to the visitor navigating the site.
They do not hold the same weight in SEO as XML sitemaps, but they are not completely without SEO value.