Usually, banks use the money of their clientele by way of using it as investments to either businesses or the stocks, which in turn, would give them some returns, while at the same time keeping the integrity of their clients' account entrusted to them. Also, by imposing interest charges on loans or other ventures, they get a premium from the money they send out, and since banks are notorious for collateral and similar means to secure the money they lend to people, it's still an advantage to them even if such people won't pay their debts because they can sequester them from other means.