As well as in the shops, for excursions, and for specialty restaurants, OBC can be used for beverages that aren’t included, in the Sanctum for services - nails, hair, massage, for Wifi, and for laundry. If it’s refundable, you can donate to the Crew Welfare fund.
I spent a fair amount in both shops on the Onward last fall and the prices I paid for a watch, a couple of Coach bags, Azamara branded men’s and women’s shirts were no more than I’d expect to pay anywhere. The quality was excellent.