From the Foreword section:
So the players have been complaining that the campaign is too dull, eh? Not enough detail? Why is this country at war with its neighbors? How can trade routes exist with all those wandering monsters? Most importantly, why do all those unsacked, unlooted dungeons and ruins stand so close to the one big city all adventurers hail from?
Rejoice Dungeon Masters, and relax! All of these questions can be answered by following the example of the volume in your hands. The World of Greyhawk is here, and it is suitable for use as a backdrop of a new campaign without changes: or, as an alternative, city country, or geographical descriptions can be used to fill in details for existing campaigns. The World of Greyhawk is in the form of a gazetteer written by a historian native to the region, and as such includes the calendar systems used by people of Oerth, a history of the major nations over the last thousand years, a discussion of climate in the different regions of the world, and even an appendix listing many runes and symbols (and their meanings) which are found in ancient and magical writings.