The purpose of Jack’s generosity with the meat was to gain control over all the boys in the group and persuade all the other boys who aren't already with him to join his group. He pretty much achieved his aim but he still wasn’t able to win over Ralph and Piggy to his group. His decision to give the meat to even Ralph and Piggy is meant to show the boys that he is a benevolent leader who will be generous even to people who do not belong in his group.