By the bankers axiom.

Buy at 2 Lend at 3 Go home at 4.

When you deposit money in a bank it isn't really in your account. The bank uses the money people have deposited in invests in things, from giving out personal/business loans and mortgages to buying stocks and shares or even buying and selling debt. The profit they make doing this covers the bank's operating cost and interest on your account.

Essentially when you put your money in a bank you are in actuality loaning the bank your money.

This is why when banks collapse people loose their savings. The bank gambled with your money and lost it all.