Although Diamond addresses this question at various times in the book, one place to find this answer is in Chapter 4. The answer starts at the bottom of p. 89 in the paperback edition of the book. There, Diamond discusses a number of different types of specialists who can be supported by a stored food surplus.
Diamond first mentions that kings and bureaucrats can be supported by a surplus of food. In other words, these are rulers and the people who carry out the rulers’ orders. Diamond then moves on to identify other types of specialists. These are people who can help a society be more powerful. These include professional soldiers, priests, artisans who can produce various types of technology, and scribes. All of these types of people do not have to work at producing food and can, therefore, do types of work that can help make a society richer and more powerful.