Simply put, protection of the eyes.
Their major three functions are:
They act as shields to prevent harmful foreign bodies from entering the eyes. Eyelashes provide extra protection against dust and sweat.
It also functions as a strong light reducer to protect the retina from intense light.
They help keep the eye covered to prevent the drying of the cornea, especially during sleep.
They also act as "mops" that spreads the tears and other beneficial secretions across the eye.
3) Vision Control:
Lastly, they act as a light input controller, allowing it to turn vision "on" and "off" for various purposes.