What purpose does each of the five factions serve in society?

In Veronica Roth's Divergent, members of society are divided into five different "factions." Each of the factions plays a role in society. Beatrice, the protagonist of the story, begins as a member of Abnegation. Qualities of Abnegation include selflessness and modesty. They function as leaders in the government because they are "regarded as incorruptible." After the Choosing Ceremony, Beatrice becomes a member of Dauntless, a faction that values bravery. Its members often have tattoos and piercings and dress in black. Their role in society is to serve as guards and protectors. Members of the Candor faction are brutally honest. They value truth above all else and dress in black and white. Candor members are the "leaders in law" because of their search for the truth. After his Choosing Ceremony, Beatrice's brother, Caleb, becomes a member of Erudite. They have an unending need for knowledge and intelligence. Within society, they function as researchers and teachers and dress in blue. The last faction, Amity, values kindness and love. Members wear brighter colors such as red and yellow. Their qualities and preferences suit them as caretakers in society. People that do not fit into one of the factions are referred to as "factionless" and live in poverty. They perform the unpleasant jobs in society that others prefer not to do. They may serve as garbage collectors or janitors.