One reason being that a level of condensed water is accumulated inside of the pot (some pots were vertical others horizontal) and maintained a fluid volume for displacement equal to or greater than the volume displacement of the instrument sensing the flow or pressure.The second reason for maintaining a liquid inside of the pot (were alsocalled chambers) was to prevent flashing of the liquid in the sensing line if a sudden temperature change of the steam is made. A dam inside of the pot prevented this flashing effect.