Ghostfire is a card that has a colored mana cost, and then is changed by its rules text to become colorless. This is largely what Devoid has been used for in editions printed after Ghostfire has first seen the light of day.
702.113a Devoid is a characteristic-defining ability. “Devoid” means “This object is colorless.” This ability functions everywhere, even outside the game. (...)
The only apparent difference is that Devoid is a keyword ability, but since it only creates a CDA (characteristic-defining ability) with exactly the same text that can be found on Ghostfire, it doesn't seem to make a difference.
Is there any ever-so-slight functional difference at all between these phrasings?
Of course, reasons other than functional ones could be, for example, that only Eldrazi-related cards would actually receive the Devoid keyword, or that Wizards maybe wants to errata as little as possible, but that's not what this question is primarily about. However if you do find citation for that, please post it as a comment, or addendum to an answer.