React provides the kind of portable isolated and testable components we wanted for our views. Turbolinks provides some of the UX we were looking for, but certainly doesn't provide isolated components that are simple to test. In addition it would mean we'd still need to decorate client side behavior with something like jQuery.
What Rails brings to the table using React? I'm starting to see a maintenance/legacy mode nearing RoR development. This is very interesting (not for railers, which I am/were)
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.
To OP: Forever will be curious about "why React" and "did you consider Turbolinks?"