Here's why:

* ATP is like a cellular energy currency: Cells store and use ATP to power their activities.

* Energy is released by breaking a phosphate bond: When a phosphate group is removed from ATP, it becomes ADP (adenosine diphosphate) and energy is released.

* Energy is used for cellular processes: This released energy powers processes like muscle contraction, active transport, protein synthesis, and many more.

While glucose is a primary energy source, it's not directly used by cells. Glucose is broken down through cellular respiration to produce ATP.