I’ve never been to zoos outside the UK, but here often the primary function is conservation - both of the animals, which are often rescues, and of wider wildlife which is funded by ticket sales
For example, London Zoo is managed by the charity Zoological Society of London [0] and places like Monkey World are essentially rescue centres you can visit [1]
> places like Monkey World are essentially rescue centres you can visit
Can confirm this (disclaimer- I'm a supporter). Many of their animals are rescues, including more than 70 Capuchins that had been lab animals in Chile and four groups of Chimpanzees, many of whom had been rescued from use in circuses or as tourist props - the latter often with teeth knocked out so that they couldn't bite the punters. There is currently a sadly growing collection of marmosets, most rescued from the UK pet trade after tipoffs from animal protection agencies. Many of the marmosets have diseases such as rickets resulting from their owners' lack of animal husbandry skills (e.g. thinking that all they need to eat is bananas). Most of these animals lack the survival skills or health to be released back into the wild. On a more positive note, Monkey World is also a hub for breeding critically endangered species, e.g. Woolly Monkeys and Orangutans.
Sadly, if an animal is being cared for at Monkey World, it generally means that the specific individual has been abused in the past and / or the species faces functional extinction in the wild.
I’ve never been to zoos outside the UK, but here often the primary function is conservation - both of the animals, which are often rescues, and of wider wildlife which is funded by ticket sales
For example, London Zoo is managed by the charity Zoological Society of London [0] and places like Monkey World are essentially rescue centres you can visit [1]
[0] https://en.m.wikipedia.org/wiki/Zoological_Society_of_London
[1] https://monkeyworld.org/