If you detect a massive ICBM launch from Russia, you know what to do - nuke Russia. You have half an hour to launch before Russian missiles take out all silos. Now what do you do if you detect SLBM launch from somewhere in the Pacific Ocean? Who do you nuke?
The US is peculiar with its nuclear policy in trying to guarantee immediate response, by contrast say the UK only has nuclear submarines at sea, and it's expected that there'll be nothing like an immediate response, rather the captains of those submarines will find out who nuked the UK and respond. So the answer for deterrence purposes is you don't need to nuke anyone right away, immediate response is only one layer…
This doctrine is called "launch on warning". It hasn't been official US policy for 20 years[1]: In 1997, the Clinton administration changed the official policy away from launch on warning to one of retaliation after withstanding an initial first strike.