Mitochondria are found in all eukaryotic cells (animals,fungi,plants,algae) and are for oxidative phosphorylation,producing energy.
Chloroplasts are found in plants and algae.They are used for photosynthesis,that's producing sugar using light energy.They give the green color to plants and algae.
It is a very good question to ask why they are functioning differently; there is no definite answer yet.But one possible explanation is from evolution which is the result of an ancient bacterial endosymbiosis - that's a bacteria that entered an eukaryotic cell.They both have a double membrane (inner and outer) which resembles the bacterial double membrane,and both have their own genetic materials.