The activity would show to who's guest book do you post. Kinda a history of your guest posts. Additionally, it could be pinned to the moderation in case of moderations(like by wallposts), so in case of getting a card, you wouldn't get left on ice. The activity would be rather private.

Like:
I check to who I made a guest book post and what did I post. I posted to user X and Y and I can see what did X get and what Y.

It could be helpful by orientation, like you want to find an old user or a meme, but forgot where did you sent it. If there is confussion, write. I can always correct the post.

Thanks for reading.