The Nemours Foundation explains that a belly button is simply a scar left behind by the umbilical cord which was removed after birth. It serves no real purpose after a human has left the womb.
The Nemours Foundation further explains that while in the womb, an unborn baby relies on its mother for nutrients to grow and survive. The baby is connected to the placenta by an umbilical cord which enters the baby in the center of the stomach, or the navel. After the baby is born, it no longer needs to receive nutrition through the umbilical cord. Doctors cut the umbilical cord shortly after birth, and it leaves a stub behind which eventually dries out, falls off and leaves behind a belly button.