In short:
- SEO - allows search engines to crawl your pages.
- Performance (initial load time).
- Cross device support.
- Same code base (sharing of logic).
- Non js users.
- Catching current trends ;)
In general, benefits for engineering, reusability, performance and SEO.
I have started to write a short recap about the subject, it's a work in progress but you can still check it out here:
https://github.com/saniko/Universal-React