Imagine needing a custom object that doesn't rely on AngularJS (such as a specific collection with unique functionalities).
You could create it independently of AngularJS and simply use it in services/controllers. Alternatively, you could design it as a service/factory to be utilized in other components.
The dilemma lies in deciding whether it's good or bad practice to develop code outside of AngularJS and then integrate it into the application. One advantage is that testing such code becomes simpler since it doesn't depend on AngularJS dependencies.
When building your apps, do you confine all code within the boundaries of AngularJS?