Why do skiers often wear sunglasses while they are skiing

They do it for personal protection, while they are skiing rocks can fly though and without personal protection the can get injured or even killed. That why they wear glasses for personal protection to avoid being injured in the eye.

For example if you are cooking and you want to take something of the oven, you would likely wear some hand gloves to avoid burns, that's the same with skiers.  

the independent variable, or the one that is intentionally manipulated is the speed.