What an HTML sitemap is
An HTML sitemap is a page on your website, built the same way every other page is built, whose content is a structured list of links to the rest of the site. It is usually grouped under headings that match the main sections: services, industries, locations, resources, company pages. Visitors can read it, click it and search it in the browser.
That is the whole difference from an XML sitemap, and the two are constantly confused. The XML file is a machine-readable list submitted to search engines. The HTML page is a human-readable list linked from your footer. They serve different audiences and neither replaces the other.
Why an HTML sitemap matters
The first reason is people. Someone who cannot find what they came for will try search, then the menu, then give up. A clear index page gives them one more route, and on a site with a lot of pages it is often the quickest route of all.
The second is accessibility. Guidance on accessible websites treats a site map as one acceptable way of offering more than one method of finding a page, alongside search and navigation, which makes it a small and cheap box to tick.
The third is internal linking. Every link on that page is a real link, so pages that sit deep in the structure or that fell out of the menu during a redesign still have a route in. It is not a substitute for good navigation, but it is a useful safety net for orphan pages that nothing else points to.
Where HTML sitemaps go wrong
The classic failure is the wall of links: every URL on the site dumped into one page with no grouping, no headings and no order. Nobody reads it, and on a large site it becomes a page of links that says nothing about how the site is organised.
The second is using it as a patch. If people need a sitemap to find your main services, the navigation is the problem and the sitemap only hides it for the few visitors who happen to reach the footer.
The third is letting it go stale. A hand-written index page is out of date the week after it is written. New pages are missing, deleted pages produce errors, and the one page that promised a complete picture gives a misleading one.
The fourth is filling it with pages that should not be indexed at all: thin tag archives, duplicated filter URLs, test pages. An index page is a strong internal signal about what matters, so it should list what matters.
How to do it well
Group the links under the same headings your navigation uses, so the page teaches the structure rather than just listing addresses. Use the real page titles as anchor text, not shortened labels, because that text tells both readers and search engines what sits behind each link.
Generate it from the content management system so it maintains itself, link it from the footer where people expect it, and keep it to pages you would be happy for a customer to land on. On a small site this takes an afternoon. On a large catalogue site it is worth building the page from the same categories your menu uses, so the index and the navigation can never drift apart.