If there was no eternal justice, than life would be an endless series of careening from one event to another, all without purpose, and all without end. Fortunately, that isn't the way that life is constructed, there is eternal justice, there is eternal morality, there is eternal love, and there is eternal forgiveness, to which we are constantly reminded of this, through that still small voice that helps to guide us in our conscience. Yet, we are endowed with free will which is a truly wonderful gift provided to us by our Creator, and free will, means exactly that, that we are free to choose, and it is through those choices and our actions that our life writes itself into the annals of time.
In this world, the unceasing opportunity to fool ourselves, consciously or not, to 'game' the system, to seek all sorts of shortcuts and illusions that we create and ultimately live in, makes us falsely believe that when it comes to that time when we stand before our Lord, that we will still be able to stand tall and thereby withstand the withering bright light of the absolute clarity of love, truth, and justice. This viewpoint will not hold true to the tests of time, and it is then, if we have not first admitted to it, that we will recognize for a certainty that the crown of glory that we wish to partake of is something that our tarnished hands cannot grasp, and we know it.
The reason for karma, is the innate understanding, that illness cannot create health, that wrong actions can never be right, that evil is not good, and thereby the false roads that we have taken cannot be effectively erased, that instead, we must first straighten up our spine, so as to thereby straighten our path to find the only true road to eternal glory. If, on the other hand, within our soul, we believed, that evil is good, that wrong is right, and tried to force this belief into the folds of our eternal destiny, we would discover that it simply cannot be done, that the bright light of eternal bliss is tainted by sin and our wrongs, so that try as we might we cannot possibly pry open the golden door.
We need karma, for the exact same reason why a helpless toddler needs both supervision and direction, which is for its own good, upbringing and protection. If, in our endeavors we are never corrected, if in our endeavors, we are unable to acknowledge the hurt that we are creating for ourselves and others, than we will inevitably continue to do it again and again. Karma is there for each of us, so that we will through thoughtfulness come to the reasoned conclusion, that the bad that we see so often in our lives is actually correlated to the bad that we have either already done or contributed to in other's lives.
The law of karma allows us to understand and to comprehend that we are all in this for the long haul, and that therefore brick by brick, stone by stone, deed by deed, we build our pathway back to God's eternal love. The race most definitely is not to the swift, to which it is to be better understood, that wrong actions necessitate correspondingly right actions. God reminds us of this, day by day, by the suffering we so often feel by the slings and arrows of outrageous fortune, to which our proper response should be to serenely arm ourselves with truth, justice, and brotherly love.