The problem was that until recently googles crawler didn't execute JS at all, it only looked at the markup received directly from the server. That meant that, t

The problem was that until recently googles crawler didn't execute JS at all, it only looked at the markup received directly from the server. That meant that, to google, your SPA looked like an empty page since it's rendered on the client. That's why being able to render your SPA on the server so you can send the markup with your first request was necessary for SEO. Google have started executing JS now though, so it's able to crawl your SPA site even if they're not server-rendered. However if you want a good ranking on other search engines other than google you still need server-side rendering.