Originally to keep rain and sweat out of our eyes. As a species we humans rely on our sight more than any other sense, and water can seriously blur vision. Eyebrows may also deflect debris and shield our eyes from the Sun. So while we slowly evolved to lose most of our body hair, our eyelashes and eyebrows remained.
But eyebrows then took on another function: communication. Facial expressions convey meaning and emotions in ways that are hard to fake, and the eyebrows exaggerate expressions. Even in cartoons, a simple line above the eyes is enough to denote anger, fear, or surprise in a face, and experiments have shown that we can recognise a familiar face more easily when the eyes are blanked out than when the eyebrows are. So if you’re tempted to redesign your eyebrows by shaving or plucking them, do remember their many uses.