Question

Why did the Constitution grant Congress the power to create "inferior courts”?

Answer 1

I believe the answer is: The builders of the Constitution ensured the extension of the principle of checks and balances.

Because of this, they separate the government branches into 3, the legislative branch, the executive branch, and the judiciery.
Creating inferior courts is a way for the legislative branch of the government to checks and limit the power of the judiciery branch of the government.

Answer 2

The builders of the Constitution wanted federal and state courts to share power under the Supreme Court.