There is a lot of fine debris floating in the air that is invisible to the eye. Such debris always clings to the surface of the eye.
The eyelids blink to remove this debris. This is the same function as the windshield wipers that remove rain and snow from the windshield of a car.
The second function is to wipe the surface of the eye with tears. The surface of the eye must always be wet. If the surface of the eye becomes wet, the surface will die and cause disease. This is why the eyelids always moisten the surface of the eye with tears to prevent the eye from getting wet. We tend to think that tears only come out when we are quiet or when dust gets into our eyes, but they are always coming out little by little.
In addition, the tears contain substances that kill germs and prevent the growth of germs in the eyes.