So if you have an external combustion thermal engine lets say a steam turbine and you need to go the superheat steam temperature in the out of steam boiler very high in order to achieve better Rankine cylce indicated thermal efficineccy and this having a maximum hot gas burner adiabatic temperature and a limited relative velocity of the fluids flow and limtided overall heat transfer coefficient,you must high the flew temperature in order to achieve a more hot steam and ever the superheated steam has a bit lower temperature than temperature of exaust gases.So you make better the efficiency of the engine and worse the efficiency of exploitation of the fuel ingition.