何时应该向私有接口添加方法?

So I've recently been using more private interfaces in my classes. I've been leaving the stuff that is needed by other classes in the .h (public), and then keeping all other methods in the private interface in the .m. My question is, I don't see any reason to keep any methods in my private interface as the app works fine without this. What is really the point? Any why aren't system methods like viewDidLoad in the private interface?So I've recently been using more private interf