Should we use transaction in spring select queries?

I saw that Spring adds Transaction(readOnly=true) to all default select queries but I couldn’t understand it, if we don’t use custom isolation levels.

Actually, my question is so clear:

First Case:

@Service
public class OrderService {

    @Autowired
    private OrderRepository orderRepository;

    @Transactional(readOnly = true)
    public List<Order> getAllOrders() {
        return orderRepository.findAll();
    }
}

Second Case:

@Service
public class ProductService {

    @Autowired
    private ProductRepository productRepository;

    public List<Product> getAllProducts() {
        return productRepository.findAll();
    }
}

Both of them use same isolation level (default mysql repeatable-read) so is there a case to get different result from these two queries?

Note: I believe that Case2 will have higher performance than Case2 because there isn’t extra transaction-manager-wrapper.