Bacteria that utilize organic compounds as electron donors, with oxygen as the ultimate electron acceptor, they produce CO2 and water as end products to release energy are:

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.