Some believe that this is to prevent rainwater and sweat from getting into the eyes when it flops down from the head, but we do not yet know if this answer is correct or not.
In the case of monkeys, an animal similar to humans, there is no eyebrow hair. Monkeys do not have eyebrows, because they have hair that extends to just above the eyes, just like the head.
In the case of humans, the eyebrows are thought to have become independent because a hairless area called the "hittai" was created between the hair above the eyes and the hair on the ears.
The eyebrows were originally part of the hairs on the hairs of the hairs of the eyes, but as evolution progressed over time, the "hairs on the forehead" were added to the eyebrows, and before we knew it, the eyebrows were left on top of the eyes.