The banking system is really most mystrious of our world because if we research deeply about all the banks in the world it has been in the control of few people so they were controlling all the money of the world so they can create as much as money they want and can pay the employees.
i think it's not a mystical thing, but it's their biggest income in terms of trading. so money they collect from customers is traded, so that they can meet their internal needs and earn a large income