It depends on the zoo. Most modern zoos work hard to give their animals large enclosures and a proper diet; those that do not do this should be shut down for humane reasons. That being said, the animals in a zoo generally live a longer life than they would in the wild, and they receive veterinary care when they are sick or injured.They are allowed to breed only when the zoo has a plan for care of the offspring.
Zoos also have become important repositories for biological diversity. Zoos worldwide have collaborated to create breeding databases so that they can work together to ensure the continued existence of endangered species in their care.
Additionally, zoos allow people to see animals first hand that they might never see in the wild. Hopefully this can help raise awareness and a feeling of connection with the planet and its creatures, so that humans will work together to maintain the earth's biodiversity, rather than destroying it. If this is true, then the animals in the zoo have performed a noble task for their entire species.