Nuking the DANKs (Dumb-Assed North Koreans): Much of the Korean peninsula, especially the north is very mountainous and rugged, making it imperative for a precise, surgical strike against DANK targets to be near 100%. Secondly, we could piss off China, who is the 800-pound gorilla in the area. And if we piss 'em off badly enough, they can hose off a couple of nukes in the direction of California or Hawaii. Thirdly, we could scare the living shit outta Japan, who can easily turn around and say 'we've got nukes for self-defense'.
If you wanna get an idea on how the US nuke war plan is created (based on some very educated guesses, since the SIOP (Single Integrated Operational Plan) is highly classified) read the following from the Natural Resources Defense Council (NRDC):
The U.S. Nuclear War Plan: A Time For Change