They are useful in customer-merchant contexts because the merchant can cancel the order after anyway in case of a double spend, and in in-person contexts becaus

They are useful in customer-merchant contexts because the merchant can cancel the order after anyway in case of a double spend, and in in-person contexts because you're in person so the risk of trying to cheat is too high. That's the main reason why zero-confs are fine; you only really need the preliminary notification to be instant, not total security.