What Diamond does to help us understand Yali and ourselves is to explain why it is that Yali (and his people) came to be so much less advanced than we are (at least in economic terms).
I believe that we in the rich world tend to see ourselves as in some way superior to those in what we used to call the Third World. This was especially true for me because I grew up in a Third World country (as an American living there) and I could see the different attitudes that my friends held about life. It seemed to me that our American attitudes were much more conducive to economic progress.
What Diamond does is to help me see the possibility that it is not culture that made my friends' culture (and Yali's) less economically successful. Instead, it could be that the geographic luck of the draw was the real determining factor. This helped me to be more humble about my society and culture and to be less judgemental about other cultures that seem in some ways "backward."