Here's why:

* Mitochondria are the powerhouses of the cell. They are responsible for cellular respiration, a process that breaks down glucose (sugar) and converts its chemical energy into ATP.

* ATP is the primary energy currency of cells. It fuels all the processes that keep cells alive, like growth, repair, and movement.

While plant cells have chloroplasts for photosynthesis (making their own food), they still need mitochondria to break down sugars and produce energy. Animal cells, on the other hand, rely on eating food for their energy and thus need mitochondria to process it.