Thermostats are a MUST when it comes to reptile heating equipment. A thermostat can make all the difference between life and death.
Not only do they control the temperature of the terraria, they also act as a breaker for when equipment malfunctions.
Thermostats work by regulating the amount of electricity delivered to a heating device in order to maintain the correct temperature. Too hot and it reduces the electricity being fed to the device, too cold and it allows more electricity through.