Intent. It says "don't call me". That might be because it might be refactored without notice, or might break the intended use of the Class. Even in languages

Intent. It says "don't call me". That might be because it might be refactored without notice, or might break the intended use of the Class. Even in languages without formal class access control, there is convention for labelling something as externally unsafe. It's an important part of sharing code.