CA were merely trying to add another kind of civilization, they had Barbs/Rome/Greek and Eastern, and wanted one more. So they put in the Mummy Returns (lol why do we always call them that, why not The Mummy, which is a better film :P) for variety in terms of civilizations, presumably to introduce more replayability.

Obviously, this was a totally stupid idea and not neccessary. But I understand where they were coming from