I'm learning about photosynthesis and cellular respiration and wonder how cellular respiration works for plants.

So if I understand correctly, photosynthesis "re-combines" the atoms so it produces dextrose and then let go of the extra oxygen and uses extra dextrose to grow. But if plants use dextrose to grow, why do they use cellular respiration at night??? Why do they convert dextrose and oxygen that I'm not even sure where it's from to water, carbon dioxide and energi? I sort of understand if they need hydration but why convert dextrose?? If they need the energi, what's the energi for? For photosynthesis???