Mitochondria are known as the "powerhouses" of the cell because they generate adenosine triphosphate (ATP), the primary energy currency of the cell, through the process of cellular respiration. They also play crucial roles in regulating metabolic processes, including the citric acid cycle and oxidative phosphorylation. Additionally, mitochondria are involved in other important functions such as regulating cellular metabolism, apoptosis (programmed cell death), and calcium storage. Overall, they are essential for maintaining cellular energy levels and overall cell health.