... holy crap is right. The effects of eight detonations are much higher than one big bomb of the equal megatonage.
Having multiple warheads gives defensive systems more threats to track and neutralize. Since each re-entry vehicle can follow a pre-programmed independent ballistic path- one missile can take out different targets. The current Minuteman III force has only 3 MIRVs per missile. Before taking their seperate firey re-entry paths they ride on a last rocket stage called the 'bus'. The Peacekeeper, which was never actually deployed, could have carried up to ten. There were rumors that the USSR had designs for a missle that could have carried 30 MIRVs!
This was all part of the chess game of targeting the other sides missile and silos. In theory a 'first strike' could wipe out the opponents missile force. Obviously both sides adopted a 'launch on warning' protocol. As soon as the early warning system of satellites and radar spotted the incoming missile barrage a massive retaliation would be launched. It was also known as the 'use them or lose them' strategy.
Both sides had (have) a 'Nuclear Triad' of silo based missiles, bombers and ballistic missile submarines. This ensures that enough of a superpower's nuclear forces will always survive to launch a devastating counterattack. This maintained the wobbly balance of Assured Mutual Destruction'. Each piece of technology being advanced and then counter-advanced across the chess board that was known as The Cold War.