For example, take the case of covering Iraq and Iran. Interceptors could be based on ships in the upper reaches of the Persian Gulf and the eastern reaches of the Mediterranean Sea. A simple survey on a globe reveals that a footprint of something like 1,000 km is required to cover Iran and Iraq from these sites. 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.