This lining is the mucosa, which swells and thickens with irritation or inflammation. The sinuses are connected to the hallway through very narrow openings, sometimes only one or two millimeters wide. The simple answer to this question is that no one knows for certain. Many theories have been proposed, all of which may be correct. By creating air-filled chambers within the skull bones, the sinuses may serve to decrease the weight of the head.
Sinuses also help warm and humidify the air that passes through the nose.