Many of the auto thermostats used depend on wax pellets which melt and expand as the engine warms up. This makes the valve open. Today, a lot of the new digital thermostats have thermistors instead of moving parts. These need batteries installed in order to operate them.
The performance of the thermostat could be affected by the water pump failing or the engine overheating or even leakage of the coolant. So it is very important to get your thermostat checked on a regular basis. Since it plays such an important part in the cooling of the engine, it must be replaced regularly, say once every couple of years, which is fine as it is a relatively inexpensive piece of equipment.
If you do not replace the thermostat when it should be replaced, you could have problems like poor fuel economy, or knocking when accelerating and your heating system may not work as well as before. Worse, you could even have a problem with emissions. So it makes sense to replace it when needed.