In the case of a conventional steam-electric power plant using a drum boiler, the surface condenser removes the latent heat of vaporization from the steam as it changes states from vapor to liquid. The condensate pump then pumps the condensate water through a feedwater heater, which raises the temperature of the water by using extraction steam from various stages of the turbine.
Preheating the feedwater reduces the irreversibilities involved in steam generation and therefore improves the thermodynamic efficiency of the system.  This reduces plant operating costs and also helps to avoid thermal shock to the boiler metal when the feedwater is introduced back into the steam cycle.