Sikyne's on the money.

Many don't realise this is an important part of how the cooling system works. Block pressure created between the pump and thermostat (or restricter) is what greatly assists the coolant picking up heat (then taking it to the radiator to be dissipated).

This is why you should never run an engine with no thermostat - the water temp reading may come down but that's simply because its not collecting heat from the engine.