That's a fantastic question... I'm not sure there's much point to this flag myself, nor have I seen it before. (Or at least I don't recall seeing it before.)
Moderators can't restore comments, so there's not much to do about the deletions themselves. The only thing to do that I can think of that could be done in response is checking to see if there are broken comment "threads" left behind as a result and cleaning them up if needed.
Shog9 and I looked at a bunch of deleted comments, users who deleted them, and the circumstances of the deletions (e.g. does deleting a bunch of comments likely point to a rage-quit in progress?), and we're gonna make some changes to how this flag works. It's only going to be raised when a user deletes old upvoted comments on 20 posts, and we're not going to show it in the 10k flag queue.
The guidance of handling it expands a bit: check the threads left behind (within reason, since there will be 20 of them), but also keep an eye out on somebody trying to take their ball and go home by deleting actually valuable and informative comments.