AIDL is used for Binder. Binder is a mechanism to do RPC calls on/from an Android Service.
When to use AIDL? When you need a Service. When do you need a Service? If you want to share data and control something in another application, you need a service using AIDL as an interface. (A Content Provider is used when sharing data only).
Services can be used within your application as the model roll in the MVC-pattern.