The body is much needed for the process of evolution.
Evolution takes place through many rebirths and taking new forms each time. (from the animal body and consciousness to the human level of consciousness.)
Thus we see different species of animals, each form has a different quality of evolution.
It is the same for humans, through probably many thousands of cycles of rebirths, we take on different forms, different roles, different race, and different circumstances. There will be a different quality of evolution each time.
The divine truth is that we all took turns to take on different roles through different cycles of rebirths. Consciousness evolved fast when your realization is fast. So, one could be rich in one a lifetime and could be a beggar in another lifetime, it also all depends on the choices and realizations in each lifetime.
As the evolution of consciousness takes place, the physical and karmic structure changed and takes on a new evolved form.
Our physical structure and intellect evolved through rebirths that aid towards achieving higher states of consciousness. Thus evolution and taking different forms through rebirths are needed for the ultimate completion.
Our state of consciousness as a modern man now has the ability to choose if we want to work towards the ultimate completion within the current lifetime or allowing the nature to take shape all by itself through the natural law of evolution and karma. We as humans at this current state can make the choice.
Some people who were born with heavy karmic issues (usually mental disabilities or less intelligent people) may have to go through more cycles of rebirths for the evolution to take place before they could consciously make a choice. Their existence were also important to help others transcend. Because without the less fortunate people, one may not see the contrast of being fortunate. It is simply how the nature of our mind and intellect works; via comparison and judgment we learn, evolve and transcend. It is just like Buddha, without seeing the sufferings of people, he would not have seek the truth, and he would not have discovered compassion.