The NIN is public for a number of reasons. This is designed to provide for an ID number without any consequences for someone to know it. The more significant number is the GIN which will intend to be between the number holder and government only. The NIN is the bank account number. This allows someone to transfer money from their bank account to another bank account with just by knowing the number. This is also the baseline for the telephone. It is also used as a private address with the postal system. And since this is a national ID number, this is also the number to be on ID cards, and therefore something the person can learn quickly, and easily.
By having a public number, it will allow the person to have a means for account numbers, and other resources without worrying about negative consequences. While this number is important, it’s not particularly powerful. Just knowing the NIN does nothing that would lead to negative consequences. This is where the GIN comes, and that is not to be made public.