Here's why:

* Glycogen is a storage form of glucose. Glucose is the primary fuel source for most cells. When a cell needs energy, it can't directly use glycogen.

* Glycogenolysis is the process of breaking down glycogen. This process occurs in the liver and muscles, and it involves enzymes that break down glycogen into glucose molecules.

* Glucose is then used for cellular respiration. This is the process where glucose is broken down to produce ATP (adenosine triphosphate), the cell's main energy currency.

In summary:

1. Cell needs energy.

2. Cell breaks down glycogen into glucose.

3. Glucose is used to produce ATP, providing the cell with energy.