One side of the moon always faces the Earth. In the event that command centers on Earth were destroyed in a surprise attack, the moon would function as a station of retaliatory strikes against the aggressor. Considering that the Earth makes one revolution every 24 hours, and that the lunar center would have a large advance notice, it could pick and choose its targets at leisure and launch an attack against which no other nation on Earth has a defense.
So, in a nutshell, deterrence.
ROFLMAO.
Care to explain how any of the above wouldn't be accomplished even more easily and cheaply from Geosychronous orbit?
BTW, we're signatory to a treaty banning nukes in space.