One of the byproducts of cellular respiration is heat. Why is it important for a living organism to generate heat through a biochemical reaction?
It's important that it does generate heat, but it's not necessary that it does generate heat. It isn't really necessary at all if you're an algae floating in the Indian Ocean, is it?
All enzymes in organisms perform in optimal conditions e.g. temperature being one of them- in the case of humans it is body temp. If the body is not in the usual range this effects the power of the enzyme as it is protein in nature and therefore it may become denatured or change shape and therefore not bind with it's substrate as well as it should and this lowers the rate of the reaction.