The mask seems to mark the difference for Jack between civilized society and a savage lifestyle. Once shed of civility, Jack is free to declare himself a leader, determine to be carefree, and decide that the beast is real whether he is real or not.
Dressed as a choir boy, Jack remained content with Ralph's rule which demanded order and responsibility. Behind a mask and colors of savage paint, Jack grew able to kill the pigs, and to eventually kill man. Jack maintained the allegiance of most of the boys by crafting an image of the carefree lifestyle that was attractive, but not responsible.
This mask and division is important because in society today, it is easy for us to likewise appear different in order to shed some of our values. In front of people we respect we often maintain our values. This piece is a cautionary tale to help us beward of the times in life when we place the mask on our face just like Jack.