Node.js has the largest repository of open source modules in the world, which solve many common and uncommon development problems ,you can find find a module for almost any functionality you are looking for, so you can concentrate to solve other problems without reinventing the wheel and boost your productivity .
This article helps you getting a basic understanding of PhantomJS .what is it ? why use it ? and how to use it ?
PhantomJS is a sort of headless browser. Basically it is the same as Chrome and Safari but without the GUI and with some additional modules to support the functionality it offers .PhantomJS is based on webkit rendering engine which is the same engine powering Chrome and Safari .So why the hell someone needs a browser without a GUI ?