Why should it matter why life exists? Do I need to know why a tree exists to know how it grows? Everything in the universe is a part of a functioning system, why this system exists in the first place is completely irrelevant. That balance is just another facet in the system, energy can neither be created nor destroyed, an animal cannot survive without taking other lives, a planet cannot emerge if stars did not decay. Sure maybe it is mystical in nature, but it is just physics.
Now where exactly would karma fit into any of this? It doesn't, in fact it flat out contradicts the system that is already in place.