Ahh haha. I've got the Youtube video that answers your question!
It's from Google I/O 2016 with Addy Osmani as the presenter.
The gist of the presentation is that he talks about building progressive apps across using three different libraries/frameworks including React.
Bottom line is that there are some pretty awesome performance gains to be had if you build your app correctly with server-side rendering.
There's also the whole SEO thing.