well, I use it on our intranet site when I have two different servers and I need to go back and forth between them. Then your URLs are http://172.22.22.2/whatever and I use <base> so I dont have to write out the http://172.22.22.2 every time.
They probably help stupid browsers and primitive robots. Plus, if all the links on a page point to another domain, say yahoo.com you can just use
<base href="http://www.yahoo.com/"> in the base tag and then in your links just use the relative path to save typing and space.