The truth is that for an extremely simple "Hello Word" all you have to do is to just simply write out the words "Hello word" and your done. But what if you later on wanted to do a little more than just to output a few lines of text? What if you wanted to create dynamic web pages with Ajax capabilities?
With PDI you get the best of both worlds. You have the abilities to easily control the elements inside your client's web browser without having to write a single line of JavaScript. But if your feel the need to roll your own Javascript code, then you can do so and use the loadScript() or addScript() methods to embedded the scripts inside the web page.