Why do insurance companies need reinsurance?

Reinsurance is essentially insurance for an insurer. That is, it is insurance which the primary insurer (one that issues policies directly to the public) buys to ensure that it has sufficient funding to pay expected claims that may be incurred during the policy period.

State insurance regulators require that primary insurers have and maintain sufficient levels of capital and reserves to pay expected claims. Depending upon the amount of capital and reserves, the insurer is permitted to issue a stated dollar amount of primary insurance. One of the ways that the primary insurer can meet the statutory requirements, other than by having all capital and reserves in cash or cash equivalents, is through a reinsurance structure that is approved by the financial authorities of the state insurance regulator.