I am not entirely sure what the use of waypoints are. I see them on many maps like East Coast USA below. I would guess that it is something to do with AI, but what? Should I be using them in my own custom map?
That's for AI mainly. AI normally uses DecalsRoad to know where it can drive. East Coast USA's town is a mesh, so we cannot use DecalsRoad on it. Instead we can manually define roads using waypoints and the map.json file in the east_coast_usa folder.