All central banks lease gold, to maintain their balance sheet and to provide sovereign collateral when a currency swap or paper trade won't work. It's called the gold carry trade.
There are currently six clearing banks on the LBMA handling gold lease transactions: Barclays, Scotia, Deutsche Bank, HSBC, JPM, and UBS all of which are primary Fed Dealer Banks, too. Central Banks need real money as collateral – physical gold holdings – to back paper (debt instruments) and as guarantor of foreign exchange sovereign liquidity, or when dealing with failed or semi-failed states.
The Bullion Banks not only guarantee and lease their own gold reserves, but require adjustments to physical gold holdings based on Geopolitical events particularly during times of war. For example, Libya, Afghanistan, Iraq, Syria, Egypt and the Ukraine have all turned over their physical gold holdings to the IMF – which acts by proxy for the Fed and G7 Central Banks – for favorable lending terms or settlement of debt to the West, or their gold is seized by force of arms.