Unity in Creation
If souls were to be mortal and rebirth a lie, there would be no need for human beings to become moral and virtuous. If human life were to be a one-off, zero-sum game, we would have all the reasons to sow all our wild oats, without caring for any rules and laws of communal life. There would be complete chaos and anarchy all around. The phenomenon of rebirth and afterlife, by keeping us on the straight and narrow path, actually keeps us motivated for being good and righteous.