why use http.createSever in express app?

I found in express api documentation, this is how one creates new express server

var express = require('express');
var app = express();

app.listen(3000);

it works properly but when you create a new app using express by default it generate following code to create new server

 var express = require('express');
 var app = express();
    http.createServer(app).listen(app.get('port'), function(){
      console.log("Express server listening on port " + app.get('port'));
    });

can someone explain me why express using http createServer method here.