Snakes shed their skin for a variety of reasons. The most commonly known reason is that, as snakes grow in size, they shed their skin to make room for this growth. Snakes will also shed because their old skin has become worn out, and is therefore in need of replacement. Shedding their skin also allows snakes to remove some of the parasites that were in their skin.
As humans, we also shed our skin. The difference between humans and snakes is that human skin is replaced in a gradual process, a few cells at a time, while snake skin must be replaced all at once.