Banks make money by using the money that others deposit with the bank (it’s more complicated than that, but that’s the ELI5 gist). If no one puts their money in a bank, the bank has no money to use to make itself money.
Let’s say there are 2 banks in town. One pays no interest to depositors, while the other pays 5% interest. Which one are you depositing your money with? That’s the only reason banks pay interest – to entice you and others to deposit their money with them.
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