There are several reasons why the eyelid blinks over our eye.
The first, and most common, cause of blinkng is to make sure that the eye itself is kept moisturized. Around the edge of the eyelid, by the eyelashes, are glands which produce a natural oil. As the eyelid moves over the eye-ball, this oil is spread across the surface of the eye. This oil also works to prevent the eye lashes from drying out too. Particularly in windy or dry atmospheres your eyelids will work overtime to prevent your eyes from drying out, one of the reasons that you may yourself blinking more on a windy day.
Another reason that you blink is to prevent foreign bodies from coming into your eye and causing you pain. In situations where the eye is at risk, the eye will blink with the aim of collecting any potential foreign bodies in the eyelashes instead.