Nostr is essentially a compromise between p2p and traditional web architectures. It cuts with the grain of the internet by using web servers, while reducing the dependence users have on servers by using keys for identity and digital signatures for authenticating data.

The effect is that users have "credible exit" (among other things), which has been discussed for years. This doesn't really create any new "use cases", which is why the use case is often described as "whatever, it's the new internet".

What it does do is introduce a very different set of trade-offs which favor user control over platform control (with the attendant UX trade-offs (or at least a different set of UX idioms)).

The reason the focus is on social is because that represents the majority of applications that do exist, the original motivation for building the protocol, and a value proposition (censorship resistance) that lots of people can relate to.