Why enzymes are necessary for many cellular reactions?

They catalyze a reaction, making it operate at several orders of magnitude faster than it would otherwise, and don't change the end products

Think of it this way, if someone were to perform surgery sure they could probably do it alone... but things go much faster if there's an assistant (enzyme) there handing them the stuff they need

Mitochondria is the cell organelle that produced the energy that is needed in the body system in form of atp, that is why it is called the power house of the cell. energy is needed by the living organisms to carry out the activities which sustain life.  if the mitochondria is destroyed atp will not be produced. if the mitochondria is destroyed, energy production will stop and the affected living organism will not have the energy to carry out the necessary life processes; this will lead to the death of the organism.