Mitochondria are often referred to as the "powerhouses" of the cell because they generate adenosine triphosphate (ATP), the main energy currency of the cell, through a process called cellular respiration. They also play a key role in regulating metabolic processes, apoptosis (programmed cell death), and maintaining cellular calcium levels. Additionally, mitochondria are involved in the production of certain hormones and are crucial for various biosynthetic pathways.