The sinuses make your head
lighter, because solid bone would weigh too much, and
would be sore all the time. The sinuses make mucus, which moisturizes your nasal passages, and cleans them, too, leaving a neat package of rubbish in your nose—boogers.
Inside the sinuses are tiny little moving hairs, called cilia, which move the mucus out of the sinuses into the nose, to make boogers, or into your throat, where it is swallowed, usually without you noticing it.