Predators naturally try to kill as many prey as they can eat. But if they could reflect on what is in their collective interest, the set of all predators on an isolated island should want to avoid killing all of the island's prey. And even in a famine, all the farmers on such an island should want to avoid eating all of their seed corn.
And even in a famine, all the farmers on such an island should want to avoid eating all of their seed corn. Human cultures naturally try to give members strong attachments to and confidence in those culture's morals. This confidence helps in fighting rival cultures.
This confidence helps in fighting rival cultures. For example, it makes members disgusted at other cultures, and so willing to fight them and not be converted to them. Also, culture members can show loyalty to each other via their confidence in its morals.