Why a command rocket that is launched to broadcast orders instead of using satellites?
I actually think the most consequential recipients are actually the land based ICBMs as those are likely 100% automated installations.
Even if the submarines are already at launch depth I would assume there would still be final humans in the loop in all nuclear bombers and subs, making them unreliable launchers. (who really wants to press the "end the world" button?)
The problem: they're nearly a single point of failure. If you can hit the other side's command systems their missiles won't be going anywhere.
I'd assume both sides know this and contingencies and redundancies exist.
Rocket silos are incredibly difficult to destroy, even with nukes.