One important difference between a mylinated and bare axon is

A myelinated axon has a sheath around it and a bore axon doesn't have it, it's open or exposed so to speak. The myelin is made up of fat and protein and works like insulation. these axons make up the white matter in the brain since it appears white. The sheath helps to keep the electrical current from leaving the axon and the myelin sheath provides a tract along which regrowth can occur. 

It allows water to resist freezing and thawing cycle.