Mitochondria are often referred to as the "powerhouses" of the cell because their primary function is to produce energy. This process occurs through cellular respiration, where glucose and oxygen are used to produce ATP (adenosine triphosphate), the main energy carrier in cells. ATP is utilized for various cellular processes such as muscle contraction, protein synthesis, and active transport across membranes.
Thus, the primary function of mitochondria is energy production in the form of ATP.