The US Constitution gives Congress the ability to create inferior courts as a means to provide federal rulings on issues between different states without having to go through the Supreme Court. Considering the size of the nation, lower federal courts are needed in order to effectively addresss all of the legal problems within the United States. Just having the Supreme Court would be extremely ineffective, as they can only hear so many cases in a year.
Partially because the Allies used submarine and air attacks to surround and divide Japanese bases, disrupt their garrisons and decrease Japanese assets-and reinforcement capacity.
