Separate the good from the bad
Sharks are the apex predators of the oceans, they have many roles which maintain a balanced ecosystem. They are able to prey on slower, diseased, infected and older fish within schools. This means that infected fish with diseases are not able to transmit the pathogens to other members of the schools, thus improving the fishes health and maintain schools numbers, removal other ill fish will also mean less transmission in future generations. Sharks will also remove older and slower fish.
Older fish could have dominance over the school, once removed they create room for new younger dominant fish which can have better chance of healthy offspring production. They also allow for the removal of other predators below them on the food chains which is essential to the survival of certain habitats. This means that primary and secondary consumer numbers are kept at a level where producers are able to sustain a balanced ecosystem.