This CANNOT BE STRESSED ENOUGH.
Fully accredited zoos are a major source of conservation funding worldwide. They take in and provide safety for injured animals. Other animals are removed from roadside zoos or even where they were kept as pets and therefore are far too people-oriented for any other life.
Fully accredited zoos are absolutely essential in breeding and husbandry of endangered species. They sometimes provide individuals of varied genetics either for release into the wild or to breed to release their offspring into the wild. They keep METICULOUS records which are incredibly important in tracking genetic diversity of small populations, which are particularly prone to inbreeding depression and other negative consequences of genetic drift.
Fully accredited zoos care about the health of the whole animal and will bend over backwards to provide the best enrichment they can with their resources. Great example: Philadelphia Zoo is city-locked and cannot expand its footprint. So they have made what are essentially giant hamster tunnels that go over the visitor footpaths. They will put novel scents or toys in there and (please someone correct me on this if I am wrong) rotate which group of big cats has access to it at any time so that when one group gets access it smells like other cats and that provides new things to explore and interact with. They have also done these tunnels for small primates.
If you want a great look at the power of zoos and what it is really like behind the scenes, all the ups and downs, check out “The Zoo” which is a reality TV show about the Bronx Zoo in NYC (similar land issues as Philly Zoo) or “Secrets of the Zoo” which is a similar show filmed at the Columbus Zoo and Aquarium and their wild animal park The Wilds. I know people who work at these zoos and who are on these shows and they aren’t making up how much care and dedication they put into their work.
Just like the relationship between some hunters’ organizations and conservation (that’s a whole other post…), the relationship between zoos and conservation is incredibly misunderstood. We need to do better.