Noss,
There is a trade-off. Allowing editing lets people correct and update things, which can be handy. But it also can make the following posts look funny when the point that they commented on has been edited or changed. By labeling the additional comment with "Edit", it leaves the original post unchanged, but also shows an update has been made. It isn't required, but it can be very helpful to let people know when you have changed something.
I use the same format when a post gets corrupted or some such thing, and I have to clean it up and re-post it - so that people will understand what has taken place. Kind of a transparency thing. It shows that I have edited someone else's post - which I hate doing unless there is no other solution. I think I have been able to keep that down to a couple times a year.