Here's why:

* ATP is the "energy currency" of cells: It's like a rechargeable battery that stores and releases energy for various cellular functions.

* Breakdown of ATP releases energy: When a phosphate group is removed from ATP, it becomes ADP (adenosine diphosphate) and releases energy. This energy is then used to power processes like muscle contraction, protein synthesis, and active transport.

* ATP is constantly recycled: Cells continuously break down ATP to release energy and then regenerate it through processes like cellular respiration (using glucose as fuel) or photosynthesis (in plants).

So, while other molecules like glucose are used as fuel sources, it's ultimately the breakdown of ATP that directly provides the energy for cellular processes.