The cuts in the lens, called fluting, shape the pattern by how they deflect the light. That's what it would be if it had a totally clear lens, with no fluting. Since the light passes through the lens, obviously the material of the lens would have an effect on how much light passes through. Lead crystal, being the clearest, allows the most light to pass through. Both ordinary glass and hardened glass are full of dirt and impurities that affect the passage of light.
Plastic lenses block even more of the light.
Most of the better lights are made with lead crystal lenses. Hardened glass doesn't break as easily, but is still as impure as ordinary glass. Plastic is basically useless for lenses because of impurities and because it gets scratched up. Breakage is, of course, an occasion for replacing lenses. And if fog lights are mounted in an air dam and you drive across the desert a lot, they get essentially sand blasted and all the little pits affect the flow of light and the light pattern.