Diamond's answer to this question is in Chapter 14 of Guns, Germs, and Steel. He claims that there are four ways that leaders justify their position and their lifestyle, which is based on their ability to extract wealth from common people. The first way is that they "disarm the populace, and arm the elite." This is similar to Max Weber's famous dictum that the state was defined by a monopoly on legal violence. It is easier today, when high-power weapons can only be produced by extremely labor and capital-intensive productions, than in former times, when weapons were simpler. Another way is to "make the masses happy by redistributing much of the tribute received" from the people. The third method is related to the first. It is to use the "monopoly of force" to maintain order. This is similar to the purpose of government as defined by Thomas Hobbes, who claimed that without government, life would be "solitary, nasty, brutish, and short." Finally, leaders can "construct an ideology or religion justifying kleptocracy."
Source: Diamond, Guns, Germs, and Steel, 277.