By having the app running on the server it can take advantage of being primed before the first request arrives. Primed in that the app is already running, and/or the data retrieved is cached and regularly refreshed.

Depending on the nature of the application, if it's largely not client personalised (a content site like Bustle, as opposed to a Gmail client), one instance of the app on the server can respond to multiple client requests in it's lifespan. So basically it's an already spun up ready to go instance of the app, or already processed, just needs to squirt the HTML buffer at a response object.