The air condition does not only controls the temperature but the humidity in the vehicle. The air condition cools down the air and the humidity is extracted from the air which will be dried.
That is why the AC is used to faster get steamed windows clear.
During Summer the humidity is not that problematic, but in Winter the windows can absolutely be frozen from the inside.
Humidity in the car can be caused by leaky door seals or the sunshine roof.
A wrong working AC can be the reason too.
If there is not enough air condition liquid, the air will not be cooled enough, condensed and causes humidity in the vehicle.
The tester shows this high humidity.
After one year without servicing the air condition's filter absorb the humidity which causes smells and bacteria.
A quick check with the 100546 shows you the information about:
1. If the measured humidity fits the producer's allowance
2. If the outpouring air is dry enough
* Normally the humidity is between 40 and 60 %
This information shows the user the AC's status and helps with a diagnosis.