Mmm so there’s 2 trade offs as far as I can see if you used a folder which both the user and app can access.

Firstly if an app does want a space that’s filesystem shape but does not want users/apps to have access for security or consistency reasons ( think Spotify offline storage of songs ).

Secondly if the user has access they can do the “easy” thing and just throw lots of files in, including things which are sensitive anyway.

It’s interesting to look at how Android and iOS have handled filesystem sandboxing in relation to this.