The nylon material on the top of the nut mechanically prevents the joint from loosening or inserting from vibration. The lock nut used to tighten the bolt during the back is elastic deformation over the thread of the screw, but the thread is not cut into it. Nylon insert locks the nut in two ways. First, forcing the bottom of the screw thread against the top of the nut thread which increases the friction between the both. Secondly, nylon applies compression against the screw itself.