What we learn about farming in Chapter 18 is that it was one of the reasons why the Europeans were able to defeat the natives of the Americas so easily when they came in contact with one another.
In this chapter, Diamond shows that the Americas had much less in the way of farming than Eurasia did. He demonstrates that having less farming meant that the Americans did not have the infectious diseases that the Europeans did. He also argues that it meant that they did not have as much technology or as complex of political systems. All of these things made it easier for the Europeans to defeat the Americans.
In this chapter, then, Diamond tells us that farming was an important cause of the proximate factors (technology, germs, political organization) that allowed Europeans to conquer the Americas.