Why we need to add

It’s the place holder for the Facebook javascript script to attach elements to the DOM. Without this when the referenced Facebook script is ran it has nowhere to attach elements.

You can see fb-root gets appended to as part of the initialisation.

<script type="text/javascript">
      window.fbAsyncInit = function() {
        FB.init({appId: 'xxxxxx', status: true, cookie: true,
                 xfbml: true});
      };

      (function() {
        var e = document.createElement('script'); e.async = true;
        e.src = document.location.protocol +
          '//connect.facebook.net/en_US/all.js';
        document.getElementById('fb-root').appendChild(e);
      }());
    </script>