React did not bring that. Flux, released a few months later, brought the unidirectional data-flow to it; we were already building similar architectures using event buses, though with looser abstractions. The 'automatic rendering' is a bit deceptive. Unless you manually optimize it, React will simply re-render everything all the time which is the same as calling `App.render()` after every data change.
> React lets developers make really smooth, really addictive websites
As someone who has been doing it for a decade, I don't see an inch of a difference. Browsers got more capable, our tools got more sophisticated, but there is nothing really you couldn't do a decade ago. If anything, React leads to bloated and inefficient SPAs.
> any flash of unstyled content, i.e. a server render round trip, lowers conversions by x%. Therefore having a client side app is imperative
There is no 'round trip' for server render, it is the exact opposite - a client-side app will have no content to show in initial render until it boots in the client. Hence you need SSR + hydration to make it faster.