I didn't mean submarines as carriers of land-attack missiles, but as anti-ship platforms. If North Korea goes to war with the ROK, it'll have to fight the S. Korean navy, or S. Korean ships will be free to attack almost with impunity. It would also be advantageous for the DPRK to be able to attack merchant shipping heading for S. Korean ports. S. Korea's effectively an island. All its trade & travel is by sea or air.
A ship hugging the DPRK coast would be of little value for that, & it'd be more expensive than land-based missile launchers, even mobile ones, & it'd be a lot of eggs in one basket. It looks to me like a vanity project for Fat Man.