Here's why:

* ATP is the universal energy currency of cells: Almost all living organisms use ATP to power their cellular processes.

* Energy storage and release: ATP stores chemical energy in its phosphate bonds. When these bonds are broken, energy is released, which can be used for various activities like muscle contraction, protein synthesis, nerve impulses, and active transport.

* Continuously cycled: ATP is constantly being broken down and regenerated within cells, ensuring a steady supply of energy.

* Derived from other sources: While ATP is the central energy source, it is derived from the breakdown of other energy sources like glucose (through cellular respiration) or fats (through fatty acid oxidation).

Therefore, while organisms utilize different fuels (like glucose, fats, etc.) as their primary energy source, it's the ATP that directly powers the vast majority of cellular functions.