Just more easy and more exact indexation of the topic/thread. About the people/users it's more easy to understand what's the url about. For example "index.php?topic=538487.msg3826887#msg3826887" is worse than "seo/about-the-friendly-url/ ", right? Well, nowadays the SEs are more advanced and the url is not that important, in my opinion and in the other SEOers opinion, but I still do prefer to use friendly urls. I hope I explained it simply and clear.