1) No stock, so no capital employed and zero risk.
2) Shipping, fulfilment and customer services are outsourced: a major drain on customer resource is removed.
3) Stock is held by the printers, so customer is neither responsible for it, required to distribute it or pay for it - until the consumer has!
4) Supply meets demand, effortlessly.
£) Provides customer with the opportunity to monetize its huge collection of images.