thermostat is nesesary for cold weather or hot weather. during cold days, it helps your engine reach operating temps by holding the water in the engine leting it get hot untill enough pressure builds up to open the thermostat. durring hot days, the thermostat help hold the hot water coming out of the engine in the radiator for long enough time for it too coll down, and when the water inside the engine is hot enough and build pressure to open the thermostat, they will switch around aND THE cool water will go into the engine while the hot one is in the radiator cooling down again. without a thermostat your car will run somewhat hotter that what it should on a hot day. and we all know that is not good