As yet more steam is generated it becomes even more compressed and its pressure continues to increase accordingly. This pressure of the steam is exerted in all directions. So besides pressing on the inside surfaces of the boiler, the steam also creates a pressure on the surface of the water.
Steam Piping Best Practices Introduction Just because it's 'been that way' for a long time, don't assume it's right. In many cases steam systems were designed long before there was any concern about energy efficiency. Systems may have not been installed