[a13n]

The two main benefits of SSR are SEO and performance.

SEO: If you don't server-side render your website, your SEO will certainly suffer. Google says they execute your JS, and to some extent they do, but your ranking will be worse and fewer pages will be scraped. We experienced this first hand.

Perf: User will stare at a blank page while they wait for your JS to download and execute. On a slow connection this could be many seconds slower than SSR.

[danpalmer]

To expand on this, Google execute your JS, but in a slightly odd JS environment (I.e. randomness isn’t random, time might not be correct, etc). They give each site a CPU time budget, and your JS execution eats into this.

If you have 5 pages that take seconds to render you might be fine, if you have 100k pages that take 50ms to render you might be in trouble.

Some of the detail of this is exposed in the webmaster tools that Google provide.