Self-insuring a prize means holding a reserve on the balance sheet against the worst plausible payout. That reserve is real capital, unavailable for anything else, until the campaign closes.
Transferring the risk to an underwriter turns that reserve into a single, forecastable line item. It also introduces a second pair of eyes on mechanic design — the underwriter will not price something they cannot verify.
Well-run finance functions treat promotional risk the way they treat FX or commodity risk: identify the exposure, decide what to keep, hedge the rest. Insured promotions are the hedge.