To protect you from outside elements, such as dust, pollen, pollution, etc. If your immune system is not working properly you may suffer from, watery eyes, a running nose, sneezing, or many other threats of sicknesses.
Histamines cause most sinus and allergy problems, and for example most people treat these problems with anti-histamines. These medications will dry the membranes and suppress the symptoms but is not a cure. Stress can often be a factor why your immune system is not working properly and, when ignored, your body cannot correct itself.
The nervous system and immune system, when working properly, can work as a natural healing aid and help to correct your sinus and allergy symptoms.