Cats have a third eyelid, or nictitating membrane, for several important reasons. First, it acts as an extra layer of protection for their eyes, especially when they're hunting or moving through dense brush. The membrane helps shield the eye from dirt, debris, and scratches. Second, it keeps the eye moist by spreading tears evenly across the surface, which is crucial since cats don’t blink as often as humans. Finally, the third eyelid contains a gland that produces a portion of the eye's protective tear film. If you notice it protruding frequently, though, it could signal health issues like dehydration or infection.