The sunglasses help take the glare from the headlights as it hits the raindrops as it reflects thru the water.
Same kind of concept with fog. You'd think high beams to illuminate the denseness. Does the opposite. You are blinded by bright white as the light reflects back. Use low beams only. (However, when I've been on roads that I know like the back of my hand that are less traveled, I use my parking lights. I can drive straight thru like butter.)