I'm also struggling to find a use case for this, and how this would go: you watch a movie while there's a little box on the screen showing your friend also watching that same content? maybe kids will like itI cannot imagine watching a show and having FaceTime on.
I am jot judging those who do, but for me it seems dry and also irritating.
Rather have the people over and spend some time together.
This feature is not something I am looking forward to. as much as continuity since I tend to use both iPad and Mac at the same time.
But I also think this would gain more traction if this were to be part of a streaming service app, instead of being system wide. Now, you need to both have compatible iOS/mac devices, the latest OS, a subscription to a streaming service that supports the feature, ... if it were part of the app both sides use, it would be easier to discover and use.