Thermostats are designed to maintain temperatures inside a set acceptable range. Temperature is typically monitored by a probe, which is placed inside your pet reptile’s enclosure. The thermostat ensures that your heating device will not overheat your reptile or amphibian's environment, which can quickly stress out your pet, and even lead to death.
Thermostats are a vital safeguard that protect your pet from extreme temperatures, and you from heartbreak. Thermostats can be used with a wide variety of reptile heating products.

Thermostats maintain temperature inside of an enclosure where as thermometers tell you what the temperature IS.

Thermostats are vital in making sure your enclosure doesn't get too hot by turning connected heaters and light fixtures off when the enclosure reaches the preset temperature  ensuring the occupants don't get stressed, injured or worse.