A monkey is not like your pet dog or cat. They are wild animals who need to live in the wild with other monkeys. They are very intelligent creatures who deserve the freedom to live as nature intended them to. Not that many years ago, people used to go and capture any animal they wanted and sell them to zoos and as pets. Now there are laws against taking monkeys and other animals from the wild.
Some people still do it and smuggle the monkeys from one country to another.
Primates are also used as subjects in scientific experiments. They are kept in cages in labs and given diseases or drugs to see what happens to them. Some people feel this is justified because it will ultimately save human lives. Others feel that there are other ways of doing research that doesn't involve animal testing.