Explanation:
To protect the animals, plants, and more. Without having the habitats they'll die/go extinct. The plants will soon rot/wither away.
Because with that the mitochondria will make ATP (energy) for the cell and your body
I just learned that today XD
I'm not sure how exactly you wanted this question to be answered. You're either talking about the symmetry where animals would have two legs or two arms for example and thus producing a pair of each muscle on both sides of the body, or you're refering to the development of agonistic and antaonistic muscles where each of them served a different purpose; either extending or contracting.