Since the U.S. has "Fiat" currency, why does the Treasury sell securities?
The sale of Treasury securities is the government's way of borrowing funds from the public. When the government sells bills, notes, and bonds, the public, the investors, become the government's creditors. The Treasury pays interest on its loans until they come due and then pays off the loans. You ask, why not print the money directly? Because, the US central bank is independent of the Treasury and it is the former that is charged with controlling the money supply, not the Treasury.