This also leave the question whether the conclusions would change when we ignore national boundaries and look at inequality among all African citizens instead.
Doing so puts the disparities that exist within African countries into context with the disparities that exist within the region as a whole, akin to the interpersonal global income distribution. Such a perspective on inequality comes naturally to an international organization as the World Bank.
One might also be interested in regional interpersonal inequality, if extreme levels contribute to issues such as migration flows. In a nutshell, we are interested, for a person living in Africa, how much of her position relative to other Africans is determined by the country she happens to be living in, vis-à-vis the position relative to her fellow country citizens?