Eyebrows are very important for diverting moisture, such as salty sweat, away from our eyes due to their distinctive arch shape. This would obviously be useful for allowing us to see clearly if it was raining or if we were sweating a lot. It is quite easy to see how this could have an evolutionary advantage in terms of escaping predators. Sweat also causes irritation in the eyes, making them sting.
Answered by Rik Sargent