Why sinuses? Who knows?

It's not entirely clear why we have sinuses, aside from their role in providing a huge revenue stream for pharmaceutical companies and over-the-counter drug makers. Some scientists think sinuses evolved mainly to lessen the weight of the front of the skull; others, to protect the eyes and brain against physical blows or rapid temperature changes; still others, to humidify air as we breathe in. An even more out-of-the-box theory holds that their role is to give the human voice more resonance.

Dr. Bill Bass & Jon Jefferson, Beyond the Body Farm: A Legendary Bone Detective Explores Murders, Mysteries, and the Revolution in Forensic Science (New York: HarperCollins, 2007), p.91