So, I was looking through the search cheatsheet and I noticed there's a metatag called hideanon, which makes posts visible or invisible for non-logged users. So I'm wondering why does this tag exists and how useful it actually is or what purpose does it have.

Maybe some posts are hidden to people who aren't logged in to to keep people from seeing things that they'd need to "opt-in" to see. I know that loli/shota on danbooru isn't visible unless you log in.

This is a feature that's extremely rarely used.
Posts with this are excluded from bots for example, so they can't be found with google, so there's couple artists who want their stuff here, but do not want to be found if that makes sense. At least this is my understanding.

So for general user this is feature you most likely never need or care of and in general you should think that this feature does not exsist, as for archive have content that cannot be searched properly is kinda eh.