Money, Banking, and Taxes
Fun fact: Fort Knox High School is one of only three Army posts that still have an on-post high school.
Security and Secrecy
There is so much secrecy surrounding this place because if everyone knew exactly what was held in the vault or how much gold was held there, there may be more problems with people trying to break in. Also the mysterious aura surrounding the Fort just makes it more important to the government. If there isn't as much gold as everyone thinks, the economy could collapse and international trade agreements could stall. Everyone counts on the gold in the vault to backup all purchases and everything else in the economy.