Suppose for a moment that you have an AngularJS single-page application, one with view routes managed with then ngRoute module, that is used by users with different roles. A user in your company's Sales group has access to certain areas of the application, while a user in Accounting works in other parts of the application. And there are also some areas of the application that are common to all users.
Now, you already have the navigation menu wired up so that users only see the navigation links appropriate to their user roles. And even if a Sales user somehow ends up in a view meant for an Accounting user, the server answering the REST calls for the data powering that view is going to check the security token sent with the request and isn't going to honor that request. But you'd still like to keep users out of UI views that aren't meant for them.
You could do a user access check at the start of each controller, or perhaps within the resolve property of each route, but that would be repetitive and it's something you could forget to do on occasion.