This is one of Gary's most fundamental ideas. The large amount of wilderness adjoining settled territory -- 25% of the land nominally claimed by nations, per Living Fantasy p. 30 -- provides space for monsters, and lots of implicit borderland where humankind and demihumankind and monsters clash, and each character class (notably the ranger whose special task it is) represents a different way of dealing with that liminal ground.
Looking at the Yggsburgh area map, it's notable how densely packed the terrain features are. Perhaps even more than they should be, but it really helps you visualize how much stuff there is in each 30-mile hex, and how many potential border zones for adventures like B2.
There was a discussion at DF a long time ago concerning Gygax's maps and layout of the land. While I always looked at the Greyhawk maps as boring expanses of huge areas (plains, desert, forest, swamp etc) with a mixture of terrains non-existant, I believe T Foster mentioned that within each hex is supposed to be a wide variety of terrain types with stream, rivers, etc. and the DM is to develop this when needed. This was something that I never new Gygax had intended until reading that thread.