Not sure what you mean by "legit use case", because you could write just about anything in vanilla, but for React specifically the value of using it comes from

Not sure what you mean by "legit use case", because you could write just about anything in vanilla, but for React specifically the value of using it comes from state management. By explicitly defining what is state and when you change it the application becomes largely pure/functional and this has a tendency to both decrease bugs and make the application easier to reason about. You could do that in Vanilla as well, but as your apps grow React or whatever.js generally is ready to grow with you, whereas your homebrew solution may not be.