Faster reaction time, less obvious launch, and ability to attack from any direction (not just the direction that your bases and submarines happen to be in). The

Faster reaction time, less obvious launch, and ability to attack from any direction (not just the direction that your bases and submarines happen to be in). There were also proposals to just launch nuclear warheads into orbit, for deorbiting in case of war, but they were banned by treaty because they were considered too destabilizing - as were partial orbital weapons, which just went into orbit for long enough to attack from unusual directions.