Also when skiing you need to be able to read the mountain - snow conditions, ice patches, ruts, slopes etc. You can't do this when the sun's out and you're not wearing something that cuts down on the sun, as it looks flat and white - you're practically guaranteed a gutser. It also stops your eyes watering on the fast bits - tear blindness isn't a lot of fun either (particularly when wearing contact lenses which then float off your eyeballs and blow round into your eye sockets
).
Anyway, it gives me a headache, too much sun. Last couple of years I've just worn normal sunglasses, £40 from Boots, IIRC. Worked fine. I've never been a fan of goggles, really.