In simple terms, PhantomJS is a web browser without a graphical user interface.
You’ll find couple of usage on internet like automation as it works with some automation tools etc and also handy for Dom manipulation, i like this tutorial #scotch.io
Before i used it to get page information on a cron and later taking screen shot and list image on CMS for A/B Testing.
its quite inserting to play on local so have a look at documentation