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. Even if you don't drive across the desert, your lenses will still be scuffed up by grit and you likely will want to replace them sooner or later, especially if you're using good quality lights. You also will care about the quality of the reflector, the shiny part that gathers the light from the bulb and reflects it out at precise angles through the lens. Some reflectors are silver plated. This is from the olden days.