Here's why:

* High-energy bonds: ATP has three phosphate groups linked together by high-energy bonds. When these bonds are broken, energy is released.

* Versatile: ATP can be used for a wide range of cellular processes, including muscle contraction, protein synthesis, and active transport.

* Rapidly regenerated: Cells can quickly regenerate ATP from other energy sources like glucose.

While ATP is the primary energy currency, cells also use other molecules for energy storage, such as:

* Glycogen: A branched polymer of glucose, primarily stored in liver and muscle cells.

* Fat: A highly efficient energy storage form, stored in adipose tissue.

However, ATP is the immediate and readily available form of energy that fuels most cellular processes.