Consider the endosymbiosis theory for the origin of the mitochondrion. How did each endosymbiotic partner benefit from the relationship?

According to endosymbiosis theory, the mitochondria was once free-living prokaryotic cell. The cell was engulfed by other cell (host cell) via the process of phagocytosis.

The "eaten" cell survived inside the host. The host cell provided a nutrition and safe environment to live and the future mitochondria produced energy that the host cell can use. Over time the organelle and the host cell have evolved together.