Here's why:

* ATP is a high-energy molecule: It stores a significant amount of energy in its phosphate bonds. When these bonds are broken, energy is released, which can be used to power cellular processes.

* ATP is a universal energy currency: It is used by virtually every cell in every organism to fuel a wide range of metabolic reactions, including:

* Biosynthesis: Building complex molecules like proteins, carbohydrates, and lipids.

* Active transport: Moving molecules across cell membranes against their concentration gradients.

* Muscle contraction: Providing the energy needed for movement.

* Signal transduction: Relaying information within cells.

While ATP is the primary energy carrier, other molecules like NADH and FADH2 also play important roles in metabolism by carrying electrons and storing energy that can be used to generate ATP.