There are two main purposes of a zoo: education and recreation. Zoological garden enables you to see for yourself the biological diversity of our planet, which makes you aware of the importance of preventing species from dying out and gives you information about the ways of saving exctincted ones.
Despite some advantages, I don't believe we have the moral right to keep animals away from their natural habitats. Artificial, to some extened, environments never will provide them(animals) fully with what they need. Another thing is that keeping animals in zoos in many cases has been proved to be both too costly and difficult to be done properly.
Mam trochę blurred vision i boli mnie głowa(alergia), także piszę szybko i może (and most likely) być tam pare błędów. (nigdy żadne zoo nie ochroniło gatunku rpzed wyginięciem-dlatego nie przetłumaczyłem poczatku)