Many developers inherit or prefer a plain page and just want to use a cool open source react component real quick like adding a slick calculator to their existing webpage.
Not to mention you can use multiple different versions of react with this.
Just as an example, I was trying to add this simple PinField to my wordpress page and it will break if you try to mount: https://www.npmjs.com/package/react-pin-field
I'd think that would be a pretty big sign that React might not be the right tool for the job. I'm all for building custom hammers but sometimes it might just be better to just grab a screwdriver.