What is the benefit of using interfaces for services in Spring MVC?

Suppose I have the following JPA repository:

public interface IGreetingRepository extends JpaRepository<Greeting, Long> {
}

And according to the documentation you would have to generate a service interface as follows:

public interface IGreetingService {

    Greeting getGreetingById(Long id);

    List<Greeting> getAllGreetings();

    Greeting saveGreeting(Greeting greeting);

    void deleteGreeting(Long id);

}

And after the implementation:

@Service
@Qualifier("greetingService")
public class GreetingServiceImpl implements IGreetingService {

    @Autowired
    @Qualifier("greetingRepository")
    IGreetingRepository greetingRepository;

    @Override
    public Greeting getGreetingById(Long id) {
        return greetingRepository.findOne(id);
    }

    @Override
    public List<Greeting> getAllGreetings() {
        return greetingRepository.findAll();
    }

    @Override
    public Greeting saveGreeting(Greeting greeting) {
        return greetingRepository.save(greeting);
    }

    @Override
    public void deleteGreeting(Long id) {
        greetingRepository.delete(id);
    }
}

And to make use of my service in my controllers I would inject it like this:

@Autowired
@Qualifier("greetingService")
GreetingService greetingService;

What I don't understand is, what is the advantage of doing it like this instead of directly injecting the JPA repository interface and saving me the 2 classes?:

@Autowired
@Qualifier("greetingRepository")
IGreetingRepository greetingRepository;