Here's why:

* ATP (adenosine triphosphate) is the primary energy currency of cells.

* Mitochondria are often referred to as the "powerhouses" of the cell because they are responsible for cellular respiration.

* Cellular respiration is a process that converts glucose (sugar) into ATP through a series of chemical reactions. This process occurs within the mitochondria.

While other organelles like chloroplasts (in plant cells) can also produce ATP, mitochondria are the primary source of ATP in most eukaryotic cells.