Additionally, as far as I know, there are two kinds of them.

Sitemaps for search engine, which usually in XML format.
And sitemaps for human /visitor. Usually it's a HTML sitemaps.

From my point of view, sitemaps will do increase user experience. However, it may be considered as a page where it contains only link (link-farm?).
To avoid such bad effect before search engine such Google, what about make that page not indexable ?

Sitemaps are simply "road maps" that search engines can use to navigate their way through the various parts and sections of a website. The sitemap essentially tells the search engines where the various pages are located on the site. This is particularly important for websites that have multi-level navigation, which automated spiders might have difficulty with.

In's and Outs of Sitemaps - Ins and Outs of Internet Sitemaps

I dont using these at all of my websites, i will try to using these

but there's an opinion that :

.html sitemaps will not increase your SEO.
Because it can be considered as a page with links only (if I'm not mistaken, the one called link-farm).
And Google hates such page.

However, such page existed on my site.
And Google crawled and indexed it.
So, personally I don't totally agree with statement : .html are use for SEO.

wow, glad to know that, since I provide HTML site maps on my site too.
I made its title as "Table of Content"