Why does Facebook use javascript for their BigPipe and Pagelets?

Facebook uses Pagelets and Bigpipes for their pages.

Source: https://www.facebook.com/note.php?note_id=389414033919

I am worried, why does they use javascript to add some pagelets into the page? Why don't they just output some part of html & flush it ?

 

its faster to load..

 

I think you didn't get me. I know that bigpipe and pagelets are designed to make loading time faster, but I don't know, why do they use their javascript in order to deliver pagelets to their place when they could just do

<?php ?>
some html here
<?php 
flush();
?>

PHP: