With a concept they call fractional reserve banking, banks practically create loans from money they do not have, debt out of thing air, they can make 100$ loans from deposit of 1$ you make and s charge interest on it
That is their biggest money maker not only they get to make money out of the money that you let with them so they keep it safe, which in my opinion is an abuse, but not only that thanks to fractional reserve banking they can loan money that they do not even have, this is why banks are so powerful because they are allowed to print money as if they were a government and they are responsible for a great deal of the inflation of the currency supply in a country.