A scratch resistant coating is often applied to surfaces that are not inherently scratch-resistant, like CR-39 plastic, polycarbonate, or Trivex. It adds a barrier of protection that hardens the surface against both large gouges and smaller scratches, which can build up and crisscross to add a hazy appearance to lenses. If your lenses are formed from any material other than glass, a scratch-resistant coating will increase their productive lifespan.