The importance Diamond assigns to writing is succinctly described in the following passage from Chapter 12, entitled "Blueprints and Borrowed Letters":
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 detail, from more distant lands and more distant times...Writing marched together with weapons, microbes, and centralized political organizations as a modern agent of conquest.
Writing enabled the conquerors to record information about the lands and people they encountered, which facilitated their rapid conquest. Diamond goes on to observe that writing was enormously difficult to invent, and spread largely through diffusion. Its main use was for record-keeping, which accompanied and facilitated the development of complex bureaucracies. As such, it remained the province mostly of elites until very recently in world history. In any case, the fact that writing, like other important technologies, spread through Eurasia relatively quickly, and not to other regions, was a result, Diamond argues, of geographic factors instead of cultural ones.
Source: Jared Diamond, Guns, Germs, and Steel: The Fates of Human Socieites (New York: W.W. Norton and Company, 1999), 215-216.