When a dynamic page is executing on the server, browser is doing nothing but waiting for the html to come from the server. If your server-side code takes 5 seconds to perform database operations on the server, then for that 5 seconds, user is staring at a blank white screen. Why not take this opportunity to download the Javascript and CSS on the browser simultaneously? By the time server finishes doing its work, server will just send the dynamic content and browser will be able to render it right away. This optimization technique can improve the performance of any dynamic page that takes some time to finish its job on the server, and has some js and css to download.
When server and browser are working simulteneously, you get to render the page significantly faster:
Server and browser working simultaneously
Read the full article here:
http://www.codeproject.com/Articles/755672/Faster-page-rendering-by-downloading-JS-CSS-before