The eyebrow may have been of use when humans had predators who attacked in the rain, or it may have been advantageous to a distant ancestor who lived through a one-off natural disaster: a meteor strike or a volcano. It may also be an effective way to communicate silently when your hands are busy elsewhere, or a vital addition to those who use fire.
The eyebrow might also have never been of any competitive advantage without also being a hindrance in any way.
Evolution is all about the death of children. If a mutation leads to the death of its carrier before they can reproduce then it will be naturally selected out of the gene pool. Anything that isn't removed in this way will still remain.
In a smaller way if a mutation helps a parent (or other relation) to raise more children to adulthood then more copies of the mutation will appear in the gene pool.
All mutations start out without a use. Maybe the advantage that an eyebrow brings has yet to be realised.