Use case: sometimes I comment on a question. Then I decide to delete it. After let's say 2 minutes the user that posted the question answers my comment but my comment is long gone! His reply to my comment is out of context now because no one can see what that comment is referring to (my deleted comment).
If I chose to delete the comment it could be hidden but stay there allowing me to bring it back. This can be done with deleted answers right now where I can undelete it if I want.