It means that you will be able to stay at 3.1, even if apple stops signing it. If you never got hashed for 3.0 or 3.0.1, you will never be able to restore to those on a 3g[s]. Unless Apple decides to stop being a bunch of butt pirates.
the 3.1 hash really doesn't do anything for you at the moment, no. If you are currently on 3.0 or 3.0.1, you are still good to go for a jailbreak though. just don't break your phone, otherwise you will be stuck having to go to official 3.1 if you don't have a spare phone to use.