Why would I need to remove duty roles from a role hierarchy?

If your custom duty roles enable actions and user interface features that your enterprise doesn't want users to perform in your application.

Note: Don't remove duty roles from predefined job or abstract roles in the reference implementation. In the Security Console, you can identify predefined application roles by the ORA_ prefix in the Role Code field. You must copy any role that doesn't match your needs, and then edit the copy.