What's the story here? Banks are buying 30-day bonds so they can borrow overnight with the Fed? They can only borrow up to the amount of the bond, so why don'

What's the story here? Banks are buying 30-day bonds so they can borrow overnight with the Fed? They can only borrow up to the amount of the bond, so why don't they just... use the money directly rather than in this roundabout fashion. If the Fed wants to keep the 30-day interest rate down, they can just buy the bonds directly themselves. They don't need to wait for the banks to buy the bonds and then repo them.