It allows you to see in a single figure the inequalities in the redistribution of a country's wealth ...
http://fr.wikipedia.org/wiki/Coefficient_de_Gini
The Gini coefficient is a number varying from 0 to 1, where 0 means perfect equality (everyone has the same income) and 1 means total inequality (one person has all the income, the others have nothing , extreme case of the master and his slaves).
For 2009 around the world this gives:
Are the Persian Gulf countries voluntarily omitted because most probably all in the red ...
You did see? Iran is better than the USA ...