Earlier quoted context omitted.
My point is that question is sort of a category error. It's like asking what type of business the internet is for, or what the use case of smart phones is. Here are a few things built on nostr, with specific use cases: primal.net is a twitter-like client with bitcoin micropayments and long-form articles (also see coracle.social, nosotros.app, jumble.social, Amethyst, Damus, yakihonne.com and many others); zap.stream…
The question isnt a category error and deserves a direct answer. If I follow what you're saying the answer could have been: "it's a framework/set of protocols for building a Twitter that can show all the stuff on other compatible Google+/Facebooks"
Nostr is a protocol that's well suited for creating decentralized applications that need publicly verifiable identity, censorship resistance and event based communication.
For example, https://zapstore.dev/app is an Android AppStore that uses nostr to provide a decentralized way to verify the developers and remove "fake" apps.