Just knew about the term “nuclear sponge”. This term is specifically very US term that refers to the 5 (or more even) US states east of West Coast states, are used as the sponge in case of global nuclear warfare. So US government put their nuclear silos there to attract their adversaries to hit that low population density states.
Using silos as bait. If the adversaries just hit the cities, risk for them to be hit back by the US using ICBM from that states. So they must hit that sponge states, don’t know how much to make silos there inoperable.