Glycogen is used as a mobile glucose storage device. The metabolism of glucose is one of the primary ways that the body gets the necessary energy for cells to function, according to "Biochemistry."
Glycogen is less reduced, and thus less energy-rich, than fatty acids, which are used for more long-term energy storage. By comparison, glycogen is more easily broken down, and is therefore important in maintaining blood glucose levels between meals. This is especially important for the brain, which uses glucose for energy nearly to the point of exclusion. Glucose can also provide energy if there is a shortage of oxygen, allowing for energy production even when cells are in a state where they are using their fuel faster than the organs can take in air, an example of this being sprinting full tilt, says "Biochemistry."
Glycogen is stored in skeletal muscles (the major movers of the body) and in the liver. While it is more densely packed in the liver, the sheer amount of skeletal muscle in a body winds up storing the majority. Glycogen in the liver is used to maintain blood-glucose levels for the overall organism, while glycogen in the skeletal muscles is used to perform the actual work of moving, according to "Biochemistry."