Research suggests that we value relative wealth more than absolute wealth. [1]

This means that inequality is important for social stability - does social stability matter?

There are possibly other reasons as well - you could argue rich people invest differently, which has implications for the global allocation of resources. [2]

[1] http://science.sciencemag.org/content/318/5854/1305.full?ijk...

[2] https://www.theglobeandmail.com/investing/globe-wealth/artic...