well we do need the energy. we need it for something. and we put it to use to do that something, we have to convert that chemical potential energy into mechanical energy etc. Also, every chemical reaction will give off some energy as heat, as no reaction is 100% efficient. actually, the efficiency of cellular respiration is only 38%. that is waay better than your car, which converts only 15% of the energy of the gasoline into movement of the wheels.