the 2 main reasons i know of are to protect the eyes form dust etc - same as the lashes, and to aid communication - eyebrows are very expressive, and help to convey emotion.
i think most animals have some form of eyebrow and they are used in a similar way - surprise, frown etc
From what I rmember from school, and joko's answer is basically correct, protection, they divert sweat and rain from the eyes, enabling us to see better for things like hunting etc,