Mismatches between NADH production and the use of ATP can stress the electron transport chain (complexes I–IV) and modulate the production of ROS. According to Moreno-Loshuertos et al., the mtDNA haplotype can also influence steady-state ROS generation in the cell. Although ROS are traditionally viewed as toxic agents contributing to cellular pathology, emerging evidence suggests that ROS are also critical in cellular homeostasis.

It's simple really. Just keep your species such as superoxide, hydrogen peroxide, and hydroxyl radical at low levels. These species function in cell signalling processes. Of course, at higher levels, these species may damage cellular macromolecules (DNA and RNA) and participate in apoptosis.