One of the reasons is probably to encourage people to think before they post something.
In a lot of other communities it's not uncommon for people to post inflammatory crap with the sole purpose of inciting others -- only to then regret it and delete all traces of what they had done.
You'll still see it here as well from time to time, where people will edit a post into a blank slate, but at least this way people can still see it was edited and judge from the responses if that user is worth communicating with in the future.
I'm sure there are other reasons as well.