1) Concerning the issue whether a zoo has a useful purpose or not, people each hold different point of views. Some people claim that 2) it has overwhelming benefits for both animals and people, while others argue that it is cruel to keep animals in cages. As far as I’m concerned, I’m in favor of the idea that 3) zoo has many disadvantageous purposes.
1) You do not have to mention the question in your introduction.
2) Are you talking about a “zoo”? or “to keep animals in cages”?
3) This sentence is kind of awkward. It would be “a zoo does not serve for its original purpose.” However, either way, you’d better mention what the purpose of creating a zoo is since zoological gardens are everywhere across countries (this surely relates to the benefits of a zoo) Therefore, you might write as follows:
People hold a different point of view in regard to the issue whether a zoo has a useful purpose or not. Some claim that a zoo has useful purposes for both animals and people in terms of animal welfare, conservation prospective, and further research on the ecosystem. However, as far as I am concerned, I am in favor of the idea that a zoo does not serve for its purposes.
Now, in your body paragraphs, you will support your ideas. For example, in your first body paragraph, you might use:
Body 1: Many animals in zoos dose not live on welfare.
Support 1: mentally stressful
Support 2: physically stressful
Support 3: ethical cruelty of animal captivity