Because people like to joke around and change thread titles to be ridiculous. E.g. someone starts a thread 'Who here owns a bike?' and in the body it just says "Please tell us below and which model".
And then a Regular user (which can be obtained without any specific personal trust) is feeling a bit mischievous waits until there are a number of replies and changes the title to "Who here is a pervert and owns a real doll?"
Ho ho, jokes, etc. The point is that you can't assume every forum is populated by people who act with complete decorum, or that people who don't fall into that category automatically have nothing of value to add to your forum otherwise.
The levels are a good idea but it would be better to have more control.
That said, on our forum it seems Leaders and Regulars get only one edit per 24 hour period so it's not as bad as it might be.