New drug and medication are often first tested on animals that may suffer similar dieases or conditions as a human. Also animal medicines have to be tested on animals in a scientic situation to determine if they are effective and safe. An example would be just about every antibiotic you can name or think of, has been tested and used on animals. Penicillin, Cleocin, E-mycin, Vancomycin etc.
Also many new surgical proceedures are first practiced on animals before being risked on a person. Veterinarians go thru nearly identical training as do medical doctors, and they do have to practice surgical techinques before they graduate. (Plus vets treat more than 1 specie!!!) heat and other organ transplants would NEVER have been done on humans with out reaserching these techinques on or "critter" friends.