Answer
As we know that human body is composed of different organ system and each organ is performing its own function different from other one. during cellular organization same cells having same structure and function combine together to form tissues. then tissues performing same function combine together to give rise to organs, different organs whose function is related up to some extent combine and give rise to organ system. Actually there is a division of labor among body cells, because many metabolic reactions are going on in our body which need such division, otherwise normal function of the body will not be performed better.
Example
Human digestive system is composed of different organs such as mouth, esophagus, stomach, small intestine, large intestine etc. As these are different organs having different composition of cells but their job is one, that is to digest food materials.
This is the reason that different body parts in different level of organization vary in complexity.