That's a good question. From the javadocs: Provides a convenient implementation of the HttpServletRequest interface that can be subclassed by developers wishing to adapt the request to a Servlet.. If there are things you find yourself frequently doing in a web app with respect to the methods of (Http)ServletRequest, you can instead extend HttpServletRequestWrapper to put those functionalities in a single place and be assured they will have been done by the time you get a HttpServletRequest in your servlet.
Looking at that particular example, the two main areas seem some support for multi-part requests (i.e., file uploads, which HttpServletRequest makes awkward to use without the help of a library like Apache Commons FileUpload) and support for MODULE and ACTION parameters (which is part of JForum's way of routing incoming requests to Java code for handling them; that ties in with the modulesMapping.properties and urlPattern.properties files). There are also some other useful extensions, e.g. the getRemoteAddr method knows about the "x-forwarded-for" header convention - something the standard implementation doesn't.