The lesson that Diamond is trying to teach us through the example of the Polynesians is that the physical environment where a group of people settles determines what sort of society they create. He uses the Polynesians to show this because they were one ethnic group with one culture that spread to many different islands with many different physical environments. After the Polynesians dispersed, they developed many different kinds of cultures depending on the physical characteristics of the places where they settled. These ranged from small bands of hunter-gatherers to relatively large chiefdoms like Hawaii or Tonga.
To Diamond, this is an important lesson. It shows that societies develop and become more complex because of the physical environments they inhabit, not because of anything that is inherent in the natures of the people themselves.