The user needs to access a specific section or functionality of a site in a quick way regardless of hierarchy.
The footer appears on all pages of your website. By adding the functionality that is vital to the concept of your website directly in the footer, you first and foremost give your users easy access to this functionality. You also mold the usage pattern of your users, by placing shortcuts to the most important functionality of your site in a spot that is always the same.
By using Fat Footers, you provide the user with an easy and natural way of browsing to a new page and while staying on your website. The Fat Footer is placed at the bottom of a page: where the user ends up, once he or she has finished reading it. Here, the Fat Footer can provide an overview of your website and inspiration for the user to continue browsing. Such information is often lost in the situation where the user has browsed to the bottom of the page, as it has been common practice only to put such links at the top of a page.
I prefer not to refer to them as a “sitemap footer” as they are a departure from the hierarchical structure of the main navigation and often a subset of what I would normally show in a sitemap.
There are scenarios on smaller sites where the footer ends up being equivalent to the sitemap but I think there is a difference between exposing select items in the footer and the full breadth that a sitemap shoots for.