In Guns, Germs and Steel, why does Diamond say writing is important for a civilization?
The answer to this question can be found in chapter twelve of Guns, Germs, and Steel. The development of writing was one of the factors that enabled European conquerors to conquer other peoples around the world, who mostly lacked literacy. This is because they could develop maps, read and write accounts of other explorers that provided information about the peoples they encountered, and other advantages. As Diamond writes:
Knowledge brings power. Hence writing brings power to modern societies, by making it possible to transmit knowledge with far greater accuracy and in far greater quantity and detail, from more distant lands and more remote times.
Of course, nonliterate people could share information too, but literate Europeans could do it more efficiently, and literacy "marched together with weapons, microbes, and centralized political organization as a modern agent of conquest." In explaining why some societies, particularly Eurasian societies, developed writing while others did not, Diamond argues that the development of writing was generally only achieved by complex, stratified societies that practiced settled agriculture. He has already spent much of the book explaining the geographic and other advantages that led some societies to develop these achievements. The point is that Eurasian civilizations did not develop writing because they were smarter, more advanced, or even that they were possessed of any cultural attributes that were more suited to the development and spread of learning. But writing was crucial to the establishment of European power over peoples around the world.
Source: Jared Diamond, Guns, Germs, and Steel: The Fates of Human Societies (New York: W.W. Norton, 1999) 215-216.