Mitochondria are often referred to as the "powerhouses" of the cell because they produce adenosine triphosphate (ATP), the primary energy currency of the cell, through the process of cellular respiration. They also play a crucial role in regulating metabolic pathways, maintaining cellular homeostasis, and initiating apoptosis (programmed cell death). Additionally, mitochondria are involved in the production of reactive oxygen species and the regulation of calcium ion levels within the cell.