Starch and glycogen molecules are a means for a plant or animal cell to store in order to maintain homeostasis.

Glucose which can also be fats from excess carbohydrates. 
Carbohydrates actually get stored in the form of long polymers of glucose molecules or as glycogen starch. They provide energy to the animals for performing their daily duties. Carbohydrates are generally considered the fuel for all organisms, and without this most animals would become sloth and would be unable to perform the regular jobs. Carbohydrates are actually very easy to metabolise and hence generate quick energy.