React is also well-suited for a single highly-interactive module on an otherwise non-React website. Of course, you still need to consider JS code size and loading performance, but it would often be completely reasonable to use React for, say, a very complex interactive form or table of data on your otherwise static Rails website.
Something like Preact can be pretty good for this too. The one thing that makes React (by itself, not with all the stupid extras) good for this stuff (as opposed to Vue and Angular) is that it has a very simple API without much surface area, so it's relatively easy to maintain, just like jQuery. But where it still falls a bit short is the actual size of the library itself, it's a bit wasteful to package in React just for a widget or something.
