Russias announcement that it is installing some of its modern, and apparently quite reliable, Topol-M ICBMs on railroad cars, indicates that they see a need for an invulnerable retaliation system. It used to be that SSBNs, nuclear subs carrying ICBMs, provided this service. But Russia knows that American SSNs (attack subs) regularly tracked Russian SSBNs, ready to destroy them before they could launch their missiles. But Topol-Ms, riding around on Russian railroads, are much more difficult to track, and attack, on short notice. The Russians need this mobile system because their missile early warning system has fallen apart. The Russians didnt really get a satellite based early warning system operational until the 1980s. Then the Soviet Union collapsed, and so did their early warning system. A few of those early warning satellites are up there, but its known that if an SSBN fires an ICBM from certain directions, the Russian warning wont pick it up. Moscow could be blown away in a surprise attack, as could many of the missile silos holding the bulk of Russian ICBMs. But the Topol-Ms on the railroad cars could fire back. Who the missiles would be fired at is another problem. But the Russians cover this by maintaining a close watch on Chinas small force of ICBMs. Meanwhile, the Chinese are rushing to complete their first modern SSBN. China has no missile early warning system to speak of, so an SSBN force given them a way to discourage someone from hitting them with a first strike. The United States early warning system has gone through several generations of equipment since the 1970s, and can spot smaller missiles, as well as some types of aircraft.
With more nations getting nuclear weapons, and ballistic missiles to deliver them with, the problem of, who attacked us? in the wake of a nuclear attack, becomes more complex, and possible. While its true that a nuclear attack would probably be preceded by some tense diplomacy and publicity, you cant always depend on that. And early warning systems, either ground or satellite based, are too expensive for most countries to justify. This has led to many nations building invulnerable retaliation weapons.