[Apostile]

if you want to get all technical, sure the thermostat keeps your coolant all locked away inside the engine block until it warms up sufficiently, but you must remember that once that temperature is reached all of that cold coolant from the rest of the cooling system has to be cycled through and warmed up. If you keep the rest of the coolant from getting unnecessarily cooled down, you'll get your engine (and all of it's coolant) warmed up quicker.