"Fraud can creep into any business. A banker, seeing other people’s gold sitting idle in his vault, might think, “What is the point of taking gold out of the ground from a mine, only to put it back into the ground in a vault?” People are writing checks against it and using his banknotes. But the gold itself seldom moves. A restless banker might conclude that, even though it might be a fraud on depositors (depending on exactly what the bank has promised them), he could easily create lots more banknotes and lend them out, and keep 100% of the interest for himself.

"Left solely to their own devices, some bankers would try that. But most would be careful not to go too far, since the game would end abruptly if any doubt emerged about the bank’s ability to hand over gold on demand. The arrival of central banks eased that fear by introducing a lender of last resort. Because the central bank is always standing by with credit, bankers are free to make promises they know they might not be able to keep on their own."

--Doug Casey