The re-birth causes regeneration, which results not in just a change in behavior, but a fundamental change of one's being. It is necessary before one can go to Heaven.
What re-birth is, and what exactly is re-born during this re-birth
What is the difference between someone with a living spirit, and someone with a dead spirit
What is death by sin, and what exactly dies
Man has three parts allowing him to die and be reborn in a body that lives only once