The main task of tears is to maintain the moisture of the eyes. Moisture is essential to maintain transparency of the cornea (the front portion of the eyeball) and to prevent ulceration.
Tears aid movement of the eyelids in blinking by lubricating the eye's surface,. Tears also wash off tiny foreign bodies that enter the eye. Tears also contain a natural antiseptic.
Of course, tears function in expressing emotion—a normal trait exhibited by humans.