I think data portability is the most obvious value proposition for most people, especially between potentially hostile services (Facebook and Google Photos could easily inter-operate, but that's not something either company wants to invest in). For someone like me, who has moved a streaming music "collection" between 4 services over the past decade or so, it would have been real handy.

It also, conceptually at least, dramatically lowers the barrier to entry for new services in the same space, since they can use the existing data models and persistence layers.

The privacy aspects are secondary, at least to start, though I think the last few years have helped establish more context for them. Providing granular controls for data access in a common format across services seems like a big win, at least compared to my experience hunting through preferences/settings/account details/etc menus in Facebook and other places.

Finally, I think there's significant potential value in this being used as way to authenticate the source of media. As deepfakes are getting better and easier to create, having all the video and audio of you connected to a known identity seems like a necessity. Letting Facebook or Google be that identity provider would be very bad.