It's important not to underestimate the value of zoos in conservation. The southern white rhino, arabian oryx, and Przewalski's horse were all extinct in the wild, and conservation efforts on zoos and private preserves have re-introduced all three of those species back to the wild. The Przewalski's horse is still listed as endangered in the wild, but the oryx is now vulnerable and the southern white rhino is not threatened. That's three species saved from extinction via zoos that I can name from a few minutes of research (technically the oryx and rhino also involved breeding programs on private preserves, but zoos played an integral part of the reintroduction).
There's also ample research opportunities available in zoos that would be impractical or irresponsible to do in the wild. This research not only improves our understanding of these creatures, but can provide highlight the importance of protecting them, and even provide insight into how best to protect them.
I won't deny that there are zoos in the world that mistreat animals, but there are also some truly spectacular ones. In the US, I highly recommend the DC Zoo, the Omaha Zoo, and the San Diego Zoo. These zoos provide ample space for their animals, and manage them using only positive feedback. In the DC Zoo, I know that medical services like vaccines for the Orangutans, for example, are entirely voluntary. They don't capture or sedate or force the Orangutans to get shots, but rather coax them with a reward (in this case, the zookeeper told me the reward is a cup of honey).