Take off your dollar store sunglasses and then put on a pair of Maui Jim’s. Then you’ll understand. This question is like asking “why would anyone buy set of headphones when the iPod comes with a perfectly good set of ear buds?” The difference in visual quality with a good pair of sunglasses is just as profound as the difference in sound quality with a good set of headphones.
As for justification…
I wear my sunglasses all the damn time. If I’m outside then I have my sunglasses on. I don’t like bright light and glare, and I can see better with them than without. Also, to those who say they aren’t bothered by glare, try driving on a sunny winter day in Wisconsin. The glare from the snow is downright painful… but not with a good pair of sunglasses.
I used to lose cheap sunglasses all the time, because there was little reason not to. Think of your cell phone. You wouldn’t care very much about losing your iPhone if you could replace it in five minutes for a dollar. On the flip side, I’ve never lost a pair of expensive glasses precisely because they are expensive. Replacing them will cost me a chunk of change, so I keep track of them.