It is to protect your eyes from things like water and sweat going into them. They may also shade the eyes from sunlight, although I think they would need to be quite big to do this don’t you?!
That’s a very good question! Eyebrows grow above the eyes to prevent sweat coming down from the forehead to enter the eyes. Sweat contains electrolytes such as sodium and potassium and when it enters the eyes it causes a stinging sensation (I wouldn’t advise you to try rubbing sweat in your eyes). So, there is a reason for our eyebrows being positioned where they are:-)