The idea is to help people get to know the world better, through a local's eye. There are hundreds of thousands of users all over the world, who have a couch free waiting just for you to occupy it, while you get to know his culture, his town in a much better way than any travel agent could show you. You can also offer your couch to other users.
Why is this good? Because we as JWs (just generalizing here, forgive me) have very little knowledge of the real world. I just read this thread: Bad Associations are the Best!! and thought how little do we actually know about 'worldlies'. Participating will most certainly help you expand your cultural horizon and learn to accept differences - invaluable for social retards like myself. Not to mention (almost) free vacation
I have just created a user on the site so far, haven't tried it out in fullest, but I think it's a very good idea. There are several layers of security if you are apprehensive, and reading some comments people most of the time have positive experience, with very small incidents of scam or such. Which also goes to prove just how bad those 'wordlies' really are.