Like elephants don't forget anything, so does not collective.elephantvocabulary. It provides a wrapper around for existing zope.schema vocabularies and make them not forget anything.
Example usecase would be a vocabulary (source) of users which from certain point in time wants to hide / deactivate some users for form or listing. But at the same time you want keep old references to user term working. This is where collective.elephantvocabulary comes into the picture. With it you wrap existing vocabulary of users and provide set of hidden list of users (term values).