From what I understand, except in some rare situations, a neck brace should be reserved for times when one is resting with relaxed neck muscles. If you use it during daily activities, it can cause neck muscles to become weak. Some suggest strengthening neck muscles so they do a better job of supporting the head.

If people think it will reduce jarring to the head from bumps in the road, the opposite is true. It will prevent the movements that absorb energy. Any jarring will be transmitted directly to the head and the joints between the cervical vertebra, especially C-1 and C-2 where there is minimal natural cushioning.