Sort:  

We can air the house any time we want when the weather is normal for summer. When the air in the bathroom is very damp after showering, it is nice to be able to open the window to get all the water vapour out quickly. If the air is cold or even something like 20 C, the air in the bathroom cools down too much, which is what you want to avoid because then there will be too much condensation in the bathroom. Powered ventilation set at a higher setting is for removing the damp air out of the bathroom quickly. But that is not necessary when the weather is hot.