[Cp1]

the real reason:

Ive operated a supra race built car for a long time without a thermostat with no ill effect on the motor however the reason it is there is allow proper heating of the motor. the colder the motor is the poorer gas mileage and greater amounts of hydrocarbons are released effectively make the t-stat an emissions control, and quite possibly the best emissions control on your car. i have a number of graphs showing engine operating temperature and emissions, so companies strive to get the motor running as hot as possible without the risk of damage to meet emissions and mileage requirements. the other main factor or result of this is that because the tolerances used to build your motor are derived directly from the desired operating temperature, ie: thermal expansion, if your car is constantly running too cold the tolerances are too far out to ensure smooth operation and transversely if you are consistently too hot... well you know what happens!