Some species in the weasel family, such as skunks, protect themselves by spraying a foul-smelling musk oil when threatened. why would a spray defense be a useful mechanism for a species?

Before the animal is harmed they spray to keep the predator away 

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.