A sauna thermometer is used to tell the temperature within the sauna. This is useful to know when trying to create either a dry or humid heat. Humid sauna requires a lower heat or else the steam be intolerable. Naturally, a dry sauna requires a higher temperature. It also provides a quick and easy way to check your sauna is at the desired temperature.
Combining a thermometer with a hygrometer is very useful, as one can calculate the optimum balance between heat and humidity for your perfect sauna experience.