Thermostats are measured for a specific temperature depending on the different types of engines. They are subject to normal wear and each thermostat is specific for each engine type.
A worn-out thermostat could stick in the open position causing the engine to run too cool and not at the correct temperature. This is uneconomical and could decrease the performance and fuel economy of your car.
A worn-out thermostat could also stick in the closed position. This will result in the engine to quickly overheat and causing possible damage.