The Gaiter keeps you warm and can help prevent a chill from creeping down your shoulders and back by insulating the neck where a rider's major arteries and veins are. You can efficiently maintain core body temperature by warming the large amount of blood that passes through this portion of the body.
It's also versatile in the fact it can be worn in different ways to cover just the neck, most of the head, or everything but the eyes like a Balaclava. You can also adjust the way you wear it over the course of a long winter ride, which is why it's a favorite piece in the Pro Peloton.
