zoos help rehabilitate animals who otherwise would not survive in the wild
zoos help protect animals that would otherwise be hunted down
and zoos give them care to keep them healthy
seaworld and circuses (involving animals)
those are the ones that are bad
Zoos, as well as reserves and national parks, have prevented thousands of different species from going extinct by overhunting by poachers, killing by predators, and helps to rehabilitate their population back to stable levels.
greatest example I would think of is the Bald Eagle, which became endangered through a number of reasons, zoos helped the ones in captivity multiply so that they could be at a stable population level when released back to the wild. and are since not under any renewed threat of dying out.
Zoos have done so much more good for wildlife and animals in general as opposed to bad, they serve a purpose, and the people who choose to work there enjoy being a part of saving wildlife across the world.