These  1) approving both changes in existing services and the establishment of new services---those are known as classification cases; 2) adjudicating complaints from anyone who believes the Postal Service is not providing rates or services as required by law; 3) issuing advisory opinions when the Postal Service proposes a substantially nationwide change in the nature of its services; and, 4) our mostly recently assigned task, providing Congress with annual reports about the costs and revenues of international mail.
It's necessary to issue advisory opinions when a nationwide change to the Postal Service is proposed.