because some of them have an injury that we people can only aid. Some species or animals in the zoo are in danger species so zoo is the right place to protect them from hunters. And one more, they changing the attitude of the people towards animals.(vise versa) and they also studying about it(their lifestyle, medicine for them and how they population grew fast). Wherein many people can see and at the sametime to give fun on them. ^_^ I wish I help you