Question

if plants have chloroplasts to harness solar power and carbon dioxide too produce sugar why do they have mitochondria as well?

Answer 1

Wikipedia says, that they are useful for the cell respiration, but also for controll of growth, mytosis and cell's death.
The main function is, that it's something like a cellular power station. In some cases they are able to produce warmth. They can also (like the endoplasmic reticulum) store calcium.