What is the best practice for creating a module?

Question

I've seen this done two ways:

1. The module is assigned a global variable which is then appended to directives, controllers, and services:

   var app = angular.module('myApp', [ ]);
   
   app.directive('myDirective' function() {
      // directive stuff here
   });
   

2. Or the the module is used as-is like:

   angular.module('myApp', [ ])
   
   .directive('myDirective' function() {
      // directive stuff here
   });
   

So why would you do it one way or the other?