Oh — so “Hide for OP” means that they don’t see the message on replies to their own posts? I misunderstood! (But I also see the use of this setting now!)
Here is the use case I have in mind. If someone chooses something like #bug or #help, there could be a link to a guide to writing good problem reports — for the first post. (This would be nice per-category in addition to a tag-based approach.)
And, perhaps replies to that (by other people, not the OP) could give a link to helpful resources for writing good answers. (But for this use case, it is the first post that is more important.)
Or, a different use case: if the topic is a news story or blog post, the text in replies could remind people to comment on the substance of the text, giving links for reporting typos or other errors (and therefore hopefully keeping the replies more helpful while reducing moderator load).