We noticed quite a few customers riding to our showroom sporting neck warmers/gators when it is cold outside. Neck warmers have been hugely popular in Europe too with virtually all the pros using them for cold-weather training. We like to use them any time the temperature drops below 50ºF. The Castelli Thermo Head Thingy is quite comfortable, warm and lightweight.
We can't say for sure if it is just an old wives' tale, but many feel keeping the neck and throat area warm helps to prevent illness. At the least it makes riding outside more comfortable and enjoyable in cold temperatures.
This cozy and stretchy garment is quite versatile and can also be used as a banana or cap in addition to a neck warmer.