Why were different breeds created to do the same job(s)?

For example, retrievers. Labrador Retriever, Golden Retriever, Curly-coated Retriever, Chesapeake Bay Retriever, Nova Scotia Duck-tolling Retriever, all bred to retrieve! Was it really necessary to have 5+ breeds created that all do the same type of job? (I have nothing against these breeds, btw! Just some examples, I could say the same for herding dogs or guardian breeds, etc.)