The military purpose of a MIRV is fourfold:
Provides greater target damage for a given missile payload. Radiation (including radiated heat) from a nuclear warhead diminishes as the square of the distance (called the inverse-square law), and blast pressure diminishes as the cube of the distance. For example at a distance of 4 km from ground zero, the blast pressure is only 1/64th that of 1 km. Due to these effects several small warheads cause much more target damage area than a single large one. This in turn reduces the number of missiles and launch facilities required for a given destruction level.
With single warhead missiles, one missile must be launched for each target. By contrast with a MIRV warhead, the post-boost (or bus) stage can dispense the warheads against multiple targets across a broad area.
Reduces the impact of SALT treaty limitations. The treaty initially limited number of missiles, not number of warheads. Adding multiple warheads per missile provided more target destruction for a given number of missiles.
Reduces the effectiveness of an anti-ballistic missile system that relies on intercepting individual warheads. While a MIRVed attacking missile can have multiple (3–12 on United States missiles and 3-10 on Russians) warheads, interceptors can only have one warhead per missile. Thus, in both a military and economic sense, MIRVs render ABM systems less effective, as the costs of maintaining a workable defense against MIRVs would greatly increase, requiring multiple defensive missiles for each offensive one. Decoy reentry vehicles can be used alongside actual warheads to minimize the chances of the actual warheads being intercepted before they reach their targets. A system that destroys the missile earlier in its trajectory (before MIRV separation) is not affected by this but is more difficult, and thus more expensive to implement.