Not only SEO. If you need to build a webapp that is also a web page (eg: blog, wiki, cms) -- you want a rich client, but also relatively plain pages -- and ideally you want to share code/templates where it makes sense.
Also to improve "bootup time", from request to usable: server-rendered templates can be cached, can provide at least visible content before javascript even has to run and can use beefy servers & the like. At that point the client "only" has to do the events binding wiring and it's done.
