Yali wanted to know why white people had so much "cargo" when compared to people from New Guinea. Germs help to answer this question because they help to explain why Europeans were able to conquer so much of the rest of the world, thus making themselves richer and more powerful still.
Epidemic diseases developed among the societies that had been farming the longest. These were Eurasian societies. Societies in the Americas, Australia, and other places had not been farming as long and lacked epidemic diseases. When Europeans came in contact with Native Americans, for example, European epidemic diseases killed huge percentages of the Native Americans. This helped the Europeans conquer and the conquest made them richer and more powerful. Thus, germs help to answer Yali's question.