A sitemap is a web page that displays the structure of a site as a structured list of links. The sitemap file can be in HTML (a web page for users) or XML (for search engines).
Thus, a sitemap in HTML format solves usability issues, allowing visitors to navigate the site.
As for the XML Sitemap file, it is not intended for live visitors, but for search robots, allowing them to quickly find new pages on the site, as well as track changes on these web pages. Thus, the XML sitemap is created for the purpose of SEO optimization of the site.
FYI
The Joomla core does not contain built-in functionality for generating Sitemap files, so you need to use third-party extensions for this purpose.