Social inequality is one of the greatest predictors of well-being on a society wide scale. At a certain point, a country getting richer has no real effect on the quality and longevity of life of its citizens. How that wealth is distributed becomes determinant, with even very rich societies having poor outcomes as the negatives of high inequality outweigh the benefits of that wealth. Conversely, much poorer countries can do much better than one would expect when wealth is more equitably distributed. People are happier, less jealous/anxious, find it easier to relate to one another, have more equitable institutions of medicine/school/work, so on and so forth. All the things we communists think are good in of themselves are actually instrumentally good too. In a strictly utilitarian sense, increasing equality makes most people (even the relatively rich!) better off. It's a no brainer to anyone with a brain.

"The Spirit Level" and "The Inner Level" written by Wilkensen and Pickett cover this very well. Highly recommend it for further reading and theory ammunition in debates with baffled libs.