I suppose the value of Rails could be the 1) the ease building a backend and API that React can then consume, and 2) delivering the initial payload and taking care (to an extent) of the various assets that are needed for this.

I've built React apps with Express as a backend, but mostly because I enjoyed doing everything myself (routing, authentication, db). For anything serious and CRUD-like, there's a decent change that I'd still pick Rails for the backend as it significantly decreases development time to create something solid.

That's not to say there aren't any alternatives. I could also pick, say, Django (Python) or Symphony (PHP). But the point is that they all bring stuff to the table that React doesn't.