Why do snakes shed their skin?

Shedding of skin by snakes:

When the snakes grow, it's outer skin layer called epidermis does not grows with it. Hence the snakes remove their skin periodically to allow the further growth of their body. In addition to this, shedding also helps them to remove parasites from their body. Before shedding, the snakes grow a new epidermis beneath the old one and secretes some fluid between the old and new epidermis. This fluid helps to seperate old epidermis from the new one. Once this process is completed, the shedding process begins. To remove the old epidermis, the snake rubs it's head against any hard surface creating a tear near the nose or mouth area, then it drags and wriggles it's body against hard surface and slowly slithers out of the old epidermis.