In the 1e DMG, it's a map. The sample of play covers about three rooms, the idea was a DM would fill out the rest. I don't really understand the obsession with it nowadays, but then again I never used it - I already had the In Search of the Unknown module by the time I picked up the DMG, and that was also a dungeon map for a DM to populate, so I got my practice in on that map. I guess for a lot of people the DotFO was their first look at a dungeon map, and so it occupies a special place in their memories, much as Quasqueton does for me, or the Caves of Chaos do for many others.