Your neck is a terrible thing to waste. That said, it's your collarbone as well as your neck that's protected by most armored collars. In fact, a clavicle, or collarbone, is the most commonly broken bone in motorcycle crashes -- when you extend your arm to break a fall, it channels the impact force directly into your clavicle. Similarly, turning a shoulder into an onrushing car, tree or street sign can break the clavicle from direct impact.
A neck collar can help you avoid the second -- a clavicle break due to direct impact. And this neck/clavicle system is the focus of all sorts of emerging high-tech protection. For example, a collaboration between two motorcycle producers, BMW and KTM, promises to stabilize the cervical spine, which is frequently and unfortunately the weak link between a strong helmet and a strong armored jacket [source: Hanlon].
On the low-tech side of neck/clavicle support is the traditional, neck-roll style collars. This contraption is like an ox yoke, and it can help pad impact and reduce the neck's range of motion in a crash.