Karma is reassuring for those who suffer by giving them a sense of comfort.
It's reassuring for those who are living quite comfortably, because it justifies all their fortune. It can even assist someone doing a crime, because they can put the responsibility on fate, and say that if they do wrong, the power that be can decide (plenty of leaders used that shtick).
So it's nice to believe. I, however, see no evidence that it is true, nor can I think of any explanation for why it would be the case. And since I don't think everybody could agree on exactly what justice is, that makes the existence of karma even more of a conundrum.
How exactly do you want that substantiated?