Why it costs less than insuring either person alone. The insurer's promise is to pay when both insureds have died, and the probability that both have died by any given year is lower than the probability that either one has. A shorter way to see it: the insurer is effectively insuring the longer of the two lifetimes rather than the shorter one, and the longer lifetime is the one that defers the claim. The gap between a survivorship premium and two individual premiums for the same total face amount is therefore not a discount the insurer is offering; it is the price of a different and later promise.
The underwriting consequence follows from the same arithmetic, and it is the reason many of these policies get written. Because the claim only arrives at the second death, a serious health problem on one of the two lives moves the price much less than it would on a single-life policy. A couple in which one person has been declined or heavily rated as an individual can often obtain survivorship coverage at an ordinary price, and some insurers will issue where one of the two is entirely uninsurable on their own. That is a genuine structural feature rather than a sales point, and it is worth knowing before concluding that a household with a health history cannot be covered.
The liquidity case, stated as mechanism rather than as a tax result. A married couple's transfer-tax exposure is generally deferred at the first death, because property passing to a surviving citizen spouse qualifies for the unlimited marital deduction and the unused portion of the first spouse's exclusion can be preserved for the survivor. The consequence is that the estate's cash obligation, where one exists, tends to land at the second death. If what the estate holds is a business, a farm or real property rather than cash, the heirs may have to sell the asset the plan exists to keep in order to pay. A death benefit that arrives on the day of the second death is cash on the date the obligation falls due, which is why the timing match, rather than the cost, is the argument for the product. The same timing logic applies to a privately held business with two owners and to a family that wants to equalize an inheritance between a child who takes over an illiquid asset and one who does not.
Two structural cautions that follow from the payout timing. The first is that nothing is paid at the first death, so a household that also needs income replacement if one spouse dies young needs separate coverage for that; a survivorship policy does not answer it. The second is ownership. Where the policy is bought to create liquidity for an estate, whether the proceeds are themselves counted in that estate turns on the incidents of ownership the insureds held, which is the reason these policies are frequently owned by an irrevocable life insurance trust rather than by the insureds. That is a separate decision with its own rules and its own three-year clock on an existing policy, and it should be made before the policy is issued rather than corrected afterwards.