If something new or relevant needs to be said, deleted comments if retraceable could be re-edited for renewed comments. Not needed often though.
Moderators can undeleted comments that were not deleted by the users. Namely, if you posted a comment and you deleted it, then moderators cannot undelete it. If, on the other hands, a moderator deleted the comment, then they can be undeleted by moderators.
The problem is that you don't have access to deleted comments, so you can't quite edit them, or restore them easily. Of course this is moot, since users can only edit comments for the first five minutes after posting them.
(Of course, any SE administrator with access to the database can undelete comments.)