But the bank doesn't just put your money in a vault and keep it there. The bank invests your money and tries to make a profit from those investments. Then it can give you back your money but keep the profit it made from the investments. That's how a bank makes money.
Suppose the investments go wrong and the bank loses money on them. Now it can't give back all your money because it has lost some of it.
It isn't just your money, but the money of all the thousands of customers who use the bank that may have been lost.
@RajorshiKoyal OK so far?