I wonder why we have sinuses? What good are they? Seriously!
I guess you mean: appart from the fact that they get to become clogged when you catch a cold? Not being a physiologist, I think it's about allowing air and fluids to pass from one region of the body to another without causing unnecessary and unhealthy pressure differences. But I think the practical real reason is to simply give you a sensation that you have a cold. Otherwise, how would you know??