Big Banks loan out that free money to businesses and citizens.
Big Banks charge interest on loaned free money, which creates more free money for banks.
When someone deposits their money in a bank, the bank takes that money and loans it out to someone else and charges interest on it; Then when that person puts the money back in the bank, the bank loans out that same money again. They can loan out the same money infinitely and make interest on all of it.
Interest and loaning literally creates more money, which causes inflation which means that all money becomes worth less because there is more and more money available.
97% of money exists only as bank deposits because banks keep duplicating money over and over again by loaning and re-loaning the same money between different people. Only 3% of money exists as actual cash. This is completely legal thanks to the fractional banking system.
The banks also take your deposits and gamble it on the stock market. When the bank loses so much through bad investments that they don't have enough to pay their balances, the government prints them even more free money, which causes even more inflation, which means that the price of living goes up; This means that everyone else is paying for the debts of the bank by having their money devalued.
https://archive.org/details/stephen-goodson-a-history-of-central-banking-and-the-enslavement-of-mankind.org