How is animal testing useful?

Today I studied a book on animal testing, I picked out little details that were about testing on animals. It talked about how cats are used for sleep mechanisms that they are suitable for tests on sleep. They mainly test on animals because they develop many of the same health problems as humans. Sometimes the scientists can do animal to human organ transplants, one time there was a heart transplant that they did but the circulation failed. Dogs are used to study effects of radiation and the first artificial hip was developed in dogs. My question is: Why do they test on Animals? They do not have all the same things as humans do. Why do they continue with animal testing when so many people think it’s so wrong?