For something unrelated, what's the point of code like this?

    <script>
        (function() {
           var hn = document.createElement('script'); hn.type = 'text/javascript';
           hn.async = true; hn.src = 'http://hnbutton.appspot.com/static/hn.js';
           var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(hn, s);
        })();
    </script>

Why not just put in the generated script tag? Moreover, why does it matter where the tag is placed if it's loaded with async as opposed to defer?