1. Culturally not needing ID is important. That's lessening, but certainly in my age group there's a considerable amount of people who do not want to be asked for ID because it's an infringement on their liberty.
2. Vote suppression. By choosing some documents (more likely to be held by your supporters) as valid ID, and others as not (make up any excuse) you can ensure your supporters aren't turned away at the polls while other party supporters are, this effectively disenfranchises them but you can pretend it wasn't on purpose.