I think if most of the platforms had their druthers private accounts wouldn't even be a thing, because to them content that not everyone can see is potentially leavin money on the table. But it's such a clear need people have that some of em end up providing it anyway, and some services like discord are somewhat built around private spaces.
Anyway, I think it's super important to have spaces where you can talk about stuff with an audience of exclusively people who already know you, rather than theoretically uh the entire world - people who know you and have all the context of you-the-person and won't assume the absolute worst of everything you say.
If you don't currently have that and you're feeling like online spaces are really hostile and alienating and designed like hellish attention zoos... well, private spaces won't fix that but they might give you some respite. It's really valuable to have a choice of spaces in which to say something - you start thinking, "Is this something I need to shout from the mountaintops? Or do I just want to say it to a few friends?" Which is a pretty baseline healthy thing, but online is weird and mostly designed by vampire capitalists. Take back your humanity from em wherever you can.
