If energy is conserved i.e. it is neither created nor destroyed, why do we need to conserve energy?
Energy is dissipated from a more usable form to a less usable form.
It is true that energy is conserved i.e. it can neither be created nor destroyed.
Energy is available in many forms. However, we can only use the ones that are in usable form.
So, we need to conserve energy which is in a usable form.