Hiding deletes the thread whereas locking it doesn't.
Eg. I can do some fishy stuff then hide the thread and the people can't find it and won't bother to complain about it should I scam them since they think they thread is deleted - I'm serious, people are stupid like this and will leave it alone.
If I lock the thread, people can still see it and it's clearly visible should anything pop up later on.
Also, if I'm a ban evader and write something in my thread and then realize it later on, I can hide my thread so people can't find it through a search. If I can only lock my thread, that wouldn't help at all since people could still find it through a search.
I know you can say people can always edit their post, but hiding a thread is still a lot quicker since it completely gets rid of the thread for everyone else and they don't even have a reference for a mod to check out. Unless the mod wants to do some digging of his own, which isn't always true, there may not be anything left.