No, its not a "required"... It means someone may have reasons not to use something, and so spec implementors need to allow for circumstances where it is not present.
Those reasons can be anything. Legal, practical, technological, ideaological. You don't know. All you know is not using it is explicitly permitted.
Those reasons can be anything. Legal, practical, technological, ideaological. You don't know. All you know is not using it is explicitly permitted.