Glycogen is basically stored glucose,  available for use by the body as an energy source. Its main storage points are in the liver and muscles.  The glycogen stored in the liver is more readily available to the body than muscle-stored glycogen. Up to 10% of the total weight of the liver can consist of glycogen. 
The hormone glucagon is sometimes used in the treatment of hypoglycemia. Its value in this is to prompt the liver to release its stored glycogen, turning it back into glucose. If the stores of glycogen are depleted, glucagon would not be useful, as there would be nothing available for the hormone to release. 
The illlustration at right shows the metabolic paths of glucose. 
- ↑ Virtual Chembook-Glycogen. Elmhurst College.
- ↑ Anaerobic Glycolysis. San Diego State University.
- ↑ 3.0 3.1 Dean L, McEntyre J. (2004). The Genetic Landscape of Diabetes. National Institutes of Health.
- ↑ Glycogen. Indiana State University.
- ↑ Insulin and Weight Gain. Mayo Clinic.
- ↑ Kimball, John. Biology Pages: Carbohydrates. Kimball, John.
- ↑ Schaer, Michael (2003). Internal Medicine. DC Academy of Veterinary Medicine.
- ↑ Illustration of Metabolic Paths of Glucose. National Institutes of Health.