You get to use all the modern tools that React gives you, an abundance of open source React components, and get an SEO and load time optimized website with minimal configuration. Building a server rendered, dynamic web app is non trivial. Next.js makes it a lot easier.
React coupled with Node.js has been used for SSR since 2015 in sites like PayPal so I'm not sure why you doubt it's veracity. React is a very strong UI library that is more than suitable for multiple rendering targets. It has never been limited to SPA apps. Facebook itself is not a pure SPA app.