What is the main source of income of bank?

Right then. A bank’s main bread and butter is the 'net interest spread'. It's pretty simple. They pay you a small amount of interest on your savings. Then they lend that money to someone else, like for a mortgage, at a much higher rate. That gap, the difference between what they pay out and what they earn, is their primary source of income. Other things like account fees are just the icing on the cake, really.