This is one of those facts that makes so much sense once you hear it. Cartography is the mapping of uncharted territories, and despite common belief, Magellan and Cortés haven't done all the work that needs to be done. With the rise of GPS units, and things like Google or Bing maps becoming so useful, there's a lot of mapping to be done in the world, and it's tempting to just steal someone else's work and pass it off as your own.
In order to prevent plagiarists from stealing, cartographers draw traps into their maps. They add things like fake streets, or fake parks and rooms. If they're found in any other map, then you know that person stole from you and it makes it easy to catch and sue them!