A "human readable" sitemap provides an alternative means for visitors to your web site to find what they're looking for. People find information in different ways. As great as your site navigation and menus might be, some people would just prefer to search or use a table of contents (or index) like this sitemap.
Human readable sitemaps are often composed of a list of your page titles, each linked to their respective web pages, as the site map on this page is. If you (or your web designer) does a good job of composing (descriptive) "page titles" that accurately describe the contents of each of your pages, then someone could preview all of the potential content on your web site quite quickly without having to wait for each page to load as they click around.
The side benefit of this type of sitemap is that you yourself get to preview all of your page titles in one place! After generating this sitemap for the first time (with this great online sitemap generator), I decided that a few of my web page titles needed improvement. Even if you don't add a site map to your web site, you can use this sitemap generator to create an "HTML sitemap" (like the map below) to at least see if you like the page titles being used on your pages. Your page titles after all, are arguably one of the most important aspects of your web pages from a search engine optimization (SEO) perspective. For those unfamiliar with "page titles", they are the text at the very, very top of your web browser, above all of the browser buttons and menus. The title of this page is: Benefit Of "Human Readable" Sitemaps (Web Site Maps).
While experts wouldn't question the benefit of an XML sitemap (which is meant for search engines to read), some would question whether a human readable sitemap is really necessary on a smaller site with good navigation. Human-readable sitemaps offer too many benefits though to simply ignore, especially on larger websites. They are of course, no substitute for well laid out, intuitive web site navigation!
One drawback of having a human readable sitemap on your website is that it becomes outdated every time that you add a new page and sometimes, even when you revise an existing page (i.e. if you revise your page title). Your XML sitemap also becomes outdated, but only the search engines read it. This means that you have an additional page to update.
Another drawback is that the online sitemap generators generate less than ideal formats (layouts) with HTML that you'll need to modify/edit to fit the map nicely into your site's theme.