For the case of covering North Korea, access to the right areas is straightforward with ship-based defense units. Ships can remain on station. Ships provide a platform that lends itself to being on station. Ships can employ large interceptors. Ships allow for employing large interceptors, and large interceptors may be required to gain the velocities needed to gain the footprints required.
Ships are mobile.
The defense battery can be positioned according to the direction of the national leaders. The one drawback is that interceptors off ships must traverse Earth's atmosphere and this poses a constraint on how fast the interceptor can go initially, until free of the atmosphere. Trajectory shaping to arrive at the target in the shortest time possible will be critical. For example, for certain countries, like Iraq and Iran, the footprints of the ship-based batteries may be much less than desired, and we must employ other concepts to cover the more remote areas of that country.