XML sitemaps are for Googlebots and crawlers. You can link to your sitemap from within your robots.txt file and Googlebot will find it automatically. They'll also give you information about indexing status of all of the links in Google Webmaster Tools.
You can also have an HTML sitemap for your actual visitors, if you'd like.
simple XML is for the Google Bots, they read them to determine all about your site and HTML is for the human beings.
More clearky it can be said as:
Google Bots considers the HTML as a page and it read the XML file to determine the site.
XML is the standard format for search engines. All the search engines and their programs are set to detect xml sitemaps for indexing. So, if you are interested in indexing your links effortlessly, you have to lead one.
The HTML sitemaps are only for better user experiences. The search engines is not gonna value these as XML.