respiration releases carbon dioxide, which plants use for photosynthesis producing oxygen than animals and humans use for respiration again, its a continuing cycle
produces energy for maintenance and repair of cells and tissues;
produces energy for muscular activities and movement;
prodices energy for nerve impulse conduction in animals and people; produces energy for light bioluminescent organisms.
im really not sure what the last one is, but i think this answer is correct
To keep you alive for one. Try google.