As we have the ability to edit our posts and replies, it could be quite important to have the ability to see the edit history of any post or even to see deleted replies as long as they have not been deleted by a moderator, for cases in which, during an argument, a user might wish to edit or delete specific replies of his own to try and undermine or change the outcome of how a discussion actually went like.
Comments edited or deleted by moderators and administrators would have their history hidden up until the staff action, which means that to edition, it would only show from the moment the moderator/administrator edit while deleted the history would be hidden to users but moderators and administrators would still have a way to find the deleted content and maybe even undo a certain action if needed.