Answer
This primarily applies to crude oil and product tankers (though applicable to older steam-powered gas carriers).
High Pressure During Cargo Operations (In Port): Tankers have massive steam demands during discharge. High-pressure steam is required to drive the turbines for the Main Cargo Pumps (e.g., Framo power packs or centrifugal pumps), stripping pumps, and the Inert Gas (IG) Blower turbines. Additionally, steam is required for cargo heating coils if the cargo is highly viscous. Therefore, the Auxiliary Boilers are fired to maximum capacity and pressure.
Low Pressure During Sailing (At Sea): At sea, there is no cargo pumping. Steam is only required for "hotel loads" and fuel heating (bunker tanks, settling/service tanks, and purifiers). This light load is easily handled by the Exhaust Gas Economizer (EGE) using main engine exhaust heat. The auxiliary boilers are shut down or kept on low-pressure standby mode. In this mode boiler pressure is low hence feed water requirement is also low which can be full filled by small feed pump as it consumes less power.