Humidification in incubators is done to ensure that the amount of water that the egg has to lose is at the required level:

Wet humidity thermometers are used for sensor calibration and accuracy in models with digital humidity display, and for in-cabin humidity measurement in older generation incubators with non-automatic and digital humidity indicators, it does NOT provide humidity control.

The thermometer, which is provided with water reservoir and special water carrier wick, can be used as dry temperature thermometer by separating it from its wick.

NOTE: For the most accurate measurement with wet bulb thermometer, the glass tube must be filled with pure water. The water carrier wick must be replaced if it becomes dirty.
It is suitable for use only in airflow and fan environment.