As author Daniel Yergin points out in the film (Episode Three: Chapter 18: The Global Divide), no economic system can survive for long without the confidence of the people who participate in it. Confidence in an economic system is shared among those who reap a benefit from it. The more widely distributed the economic benefit, the greater the legitimacy of the system and the stronger it stands. The distribution of benefits thus has political as well as material ramifications.
Economists track distribution within an economy by dividing the population into groups of equal size, and looking at the share of total income each group receives. In a totally egalitarian society, each same-sized group would earn an equal share of total income. But in the real world, that's not what happens.