Nose is a serious filter, pre-heater and humidity conditioner for the air. It is full of hairs that trap larger particles so that these do not get through it, and it moves air through the system of sinuses to make sure it is warm and humid enough. If there is enough air passing through the nose, it is healthier for other parts downstream to have this filter machinery engaged. This is the most true if it is cold, dusty or very dry.