Like a lot of people in this Twitter thread, I didn't really understand that React Native was even for building on the web. I thought it was a way to write React to build native mobile apps. Nicolas has a whole "React Native for Web" repo though, explaining otherwise. Plus a conference talk.
Several of the replies suggest "a better DOM" which is interesting. Or, as Nicolas points out, it's kinda like "web: the good parts" as much of it is an intentionally limited subset of the involved platforms in order to simplify things and make them interoperable.
Obviously, this isn't for every project. But, if you have a React-based website already, and either have or want a native mobile app, then it seems like this is worth exploring.