Before SSR (unless you were using PHP I guess) you had to ship a shell of a site with all the conditionals being decided only AFTER the browser has gotten all the HTML + JS pulled down. If you need to make any API calls, you've delayed rendering by hundreds of milliseconds or worse (round trip to your server)

With SSR, those round trips to the server could be down to single-digit milliseconds assuming your frontend server is in the same datacenter as your backend. Plus you send HTML that has actual content to be rendered right away.

A truly functional pageload can go from seconds to milliseconds, and you're transferring less data over the wire. Better all around at the expense of running a React Server instead of a static file host.