Basically, you want to be able to control the quantity of money, because it's a key element of monetary policy. Monetary policy is a tool that governments use

Basically, you want to be able to control the quantity of money, because it's a key element of monetary policy. Monetary policy is a tool that governments use to counter boom and bust cycles, so economic cycles are smoother, and to maintain price stability. Under a metallic standard, monetary policy is much more limited. As far as I know, a central bank can still sterilise inflows of money that result from a trade surplus with the rest of the world, and release its reserves, as a way to influence the quantity of money, but it's a lot harder.