predators keep the populations in check. most "prey" animals breed way more than is necessary to maintain a healthy population. predators serve the function of keeping the numbers low enough such that the prey species have sufficient room and food to survive. otherwise they would breed themselves out of food and space.
sabertooth tigers arose when their prey animals were getting too numerous, and they vanished when their prey populations got small.
sharks are the same way. they don't evolve. they just cull back the herds.
they also kill off the weakest members of a species so only the strongest ones survive long enough to breed and pass their dominant genes along to the next generation thus advancing the evolution of the species.