And you all thought he was sporting them to hide his ravaged and middle aged visage, sculptured out of excess pints of Guinness, Middleton whiskey and that sad fact that he's a well known proverbial:

http://www.telegraph.co.uk/news/celebritynews/11169267/Bono-reveals-he-always-wears-sunglasses-because-of-glaucoma.html

Still, it doesn't answer the mysterious question as to what is behind's the Edge's hat?