The DOM and all additional resources don't need to be parsed, the client can already begin the download of the `screen.css` resource, without "wasting time" processing the DOM and all external resources, only to make a new request to the server to begin fetching them. When you add this all up for all resources on a page, this can easily save 100-200ms of the total page load/paint of a website. Those are numbers that should really have you consider implementing HTTP/2.

http://ma.ttias.be/service-side-push-http2-nghttp2/