Often, social concepts and even vocabulary is vague. Arguably, Cambodia had greater 'equality' between citizens in 1978 than the United States now has. I doubt, however, that many would consider their means or ends desirable. In order to explore salient social issues a common ground, with a common language, is necessary. Regarding inequality, we might devise a poverty threshold.
Poverty rates are something most people understand.
Poverty rates are by no means perfect, but at least when we talk about a 'poverty rate' we all tend to understand what we mean when we discuss poverty. Some argue that, in order to truly understand a social phenomenon, the researcher should be value-free or neutral. The proponents of this view argue that once a researcher becomes personally involved he or she loses their perspective. They become biased. Those biases influence their study of society.