"Think" is the wrong word. I accept the evidence (http://www.equalitytrust.org.uk/why), just like I accept the evidence for gravity and light. The book that presents that evidence (http://www.equalitytrust.org.uk/resource/the-spirit-level) is a good read on the topic.

What exactly are we aiming to accomplish?

The Spirit Level cuts beyond the rhetoric and presents what happens when societies are/are not more equitable. Importantly, it doesn't matter why they are what they are. Japan has low taxes, but they have a cultural consensus on equity, so they experience very similar outcomes as, say, Denmark or Sweden.