or currency is the debt of the central bank (called Federal Reserve Note in the U.S., check your dollar bills). They are the liability (debt) of the issuing bank. To issue debt the bank needs collateral asset. In the past the Fed mostly issue money backed by the treasury debt. This past year they have been using mortgage collateral as well. In the good old days, gold were collateral. How much cash Fed will issue is dependent on the interest rate it wants to target. If it buys all the treasury debt with its own debt, there will be instant hyperinflation. There will be so much money in circulation that money will literally not be worth the paper they are printed on.
The privilege of the issuing bank is that currency is a zero interest debt, while they may own collateral that pays interest. So they are all money making machines. The profit can also be viewed as mostly a form of inflation tax.