Some of the sun's UV rays can be absorbed by the Earth's surface or bound back off. Although this would be a "good" thing, it is not. These very rays cause the ozone layer to deplete over long periods of time from the rays "breaking it.
Summers are upon us and thanks to global warming it is getting otter and hotter each year. The sun rays are more torturous now a days due to the ozone layer depleting. But to protect your pretty eyes from all these unwanted troubles you got shades!