How is Django's classonlymethod useful? [duplicate]

Question

The source code is below:

class classonlymethod(classmethod):
    def __get__(self, instance, owner):
        if instance is not None:
            raise AttributeError("This method is available only on the view class.")
        return super(classonlymethod, self).__get__(instance, owner)

Though I can see that classonlymethod can only be called on the class and not on an instance unlike classmethod of python, why do we need such a "restriction"?

Not much on the www regarding classonlymethod and any layman examples appreciated as always.

Answer 1

It is used internally on class-based views' as_view to give a descriptive error message to people who try to call it on an instance.

I'm not sure who first decided it's mandatory.