Can someone explain me what is so special about EmberJS Fastboot?

There is a lot of buzz around EmberJS' Fastboot and I have been reading about it here.

However, I haven't gotten the whole idea:

1. What does it mean to "boot JavaScript apps on the serve"?


2. Why should it be done?

I have no experience in writing web applications. Nevertheless, I know that the server side handles requests from client by sending a JSON (e.g.) response. Then, the client takes it and mainly wraps it with HTML & CSS.

So why do I need to boot my JavaScript app?

Thanks.