To survive, organisms need a source of energy. There are two basic ways to obtain energy: synthesizing it (producing) or taking it from the environment (consuming). Organisms that are "producers" are called autotrophs, and "consumers" are heterotrophs. Autotrophs produce energy-bearing nutrients from simple molecules. They power this production from light energy (organisms that do this are called photo-autotrophs) or energy from other chemical reactions (in chemoautotrophs). Then, when the organisms need energy, they can consume the nutrients they synthesized. Heterotrophs cannot produce energy-bearing nutrients like autotrophs, so they have to get it from autotrophs (or other heterotrophs).
Autotrophs need mitochondria because it is the site where the cellular respiration process occurs. The respiration process is essential in the production of energy. As a result, autotrophs, being organisms that make their food, they require energy to do so.