The original way to minimise risk
Voyages were written on a board and individuals willing to take on a portion of a voyage's risk would write their names under that voyage, and the percentage of the risk they were willing to accept - hence the term "Underwriter."
Although it is slow, modern sea transport is a highly effective and relatively cheap way to move non-perishable goods. Nearly 35,000 commercial vessels carried 7.4 billion tons of cargo in 2007 and 80% of the world's goods move over the seas.
Imagine for a minute a world without shipping insurance. Instead of cotton, lycra and denim we'd be wearing leather trousers, woolly jumpers and a fox fur coat, topped off with rabbit ear muffs. Nice!