It seems to either be homeostasis so the snake may regulate its state of being like how a person will take off their coat when it gets too warm or growth where the snake sheds its skin to prepare for growth. From what I can tell, the answer can't be excretion, reproduction, or nutrition. As the skin, gave no initial nutritional value so that eliminates excretion and nutrition. The snake isn't giving birth to other snakes during this process so that eliminates Reproduction.