[lazopm]

It's a way to preemptively send assets to the client before they request them.

[blattimwind]

Pretty much the same effect is granted to any HTTP version by simply including stylesheets or scripts verbatim into the HTML.

[Piskvorrr]

That's anything but "simple" - you might want to reuse those stylesheets/scripts on other pages as well, for example; if you inline them into HTML, you're now wasting far more bandwidth, as you're unconditionally pushing them with every HTML response.