Found my way into a great #FediForum discussion on implementing the ability to create private groups, and it startled me how emphatically a few folks were puzzled why this would be desired. One person repeatedly characterized private groups as "hiding from the public square". It feels like issues of harassment (sexism, racism, homophobia, transphobia, etc.) are talked about with such urgency (at least in my circles?) - I can't be the only hearing that? The public square isn't safe for everyone.
It doesn't even have to be about safety; you can just want a closed group period - a way for you & your friends/classmates/colleagues to be together without strangers peeking in and eavesdropping on your conversation.
If you are a study group (for example), you want it to be just you and your study mates; no one else is welcome and it wouldn't be appropriate.
I would love to have something like this in the Fediverse instead of trying to use cumbersome and/or expensive commercial options.