They're lenses for glasses that get darker or lighter in varying light. So in extreme light, they'll get darker and act as sunglasses while retaining your prescription, so you don't have to take off your glasses or whatever while outside in favor of actual sunglasses. They were just made so people who need their glasses don't go taking them off to have sunglasses on while they're doing something important and need to be able to see, being able to keep your glasses on all the time so your vision doesn't deteriorate, and just for convenience.