Both :
app.listen( port, function () {
  .. doing stuff ..
});
module.exports = app;
and
http.createServer(app).listen(port)
Will do quite the same. What's the main difference ??
Well, module.exports here is just for testing or extending proposes, so you can require app in your test and work with it. In another hand, app.listen is much shorter and cleaner syntax.
