It is thought that the length, thickness and distribution of body hair in humans is for evlutionary benefit. Humans retained plentiful hair on the tops of their heads for protection. This makes sense since the head is one of the main parts of your body that’s consistently exposed to the sun. That means there’s a greater amount of heat and radiation that reaches it directly. Head hair protects our head from the burning sun and prevents bacteria from getting onto our face from the top of the head.
As humans evolved, the overall amount of human body hair diminished.
My guess is that eye lash and eye brow hair protect the eyes from dust/germs, but remain fine so as not to interfere with vision..Some people tend to have pretty thick brows and lashes though, and this just genetics now, some ethnic groups tend to be hairier than others.