Question

Why did Truman want to pass the National Security Act of 1947?

Answer 1

It consolidated the oversight of the military branches and established formal foreign intelligence and analysis organizations.

Explanation:

Post-WW-II, the military branches were quite independent, and there was a lot of duplicated and sometimes contrary action in intelligence and operations. The formation of a Secretary of Defense over all of the military branches and centralized agencies (the CIA and NSC) for intelligence gathering, analysis and foreign contacts streamlined those activities and advised the President directly.
https://history.state.gov/milestones/1945-1952/national-security-act