I really can't ses a single use case for this.
I never ever had a moment where any friend wanted to watch me play a game on the edge of his screen or vise versa.
the Xbox One had a very similar feature that was so rarely used by anyone that they ultimately killed it in order to free up resources for more liked, wanted and used festures
When it comes to gaming I'm very anti-social, so I never ever going to touch any of those social media features, but Its good for people who care.
It's not for me and I can't see myself ever using it, however I think it's a cool and innovative feature that I'm sure many others will use
(I don't play online/multi-player or do anything social at all when gaming, 100% solo)
1 device (TV) with 2 screens is much easier than watching on 2 different devices.
These additions I've seen so far on PS5 are great QOL additions. I don't know why or how anyone would not want them. Anything that makes the user experience much easier, simpler and seamless is welcome.
I also play only singpleplayer, never had PS Plus and even my flatmate who has a PS4 I don't think we are even friends for chatting, etc lol mostly because we'd just use discord or shout through the house (I'm trying to stop doing this, I'm getting better lol), we have no women in the house so the WAF is currently not considered, if it was it would be permanently set at 1000 (The maximum possible value).
Anyway, my point is that I will now use this feature to bother my flatmate into viewing every thing I find exciting in my game, no longer will this scenario occur:
Kuranghi: "Hey mate, come and check THIS out, its amazing"
Flatmate: "Begrudgingly gets up and comes in to see what I'm trying to show him"
*comes in to find me showing him an out of context event from my game that is confusing to him and requires too much explanation*
Now I can just send him constant, irritating requests to view my mostly personally meaningful to me only, in-game moments!
He won't be able to escape! Muhahahaha!
I also like the idea of being able to give more subtle hints to people via this, by just showing them in-game how to solve/start solving a puzzle. Instead of a spoilerific description of what to do, I know some people are shit at giving hints.