My guess is that the more detailed, dense map allows the car to precisely localize itself by using its view of the world to match against the map, yielding a lo

My guess is that the more detailed, dense map allows the car to precisely localize itself by using its view of the world to match against the map, yielding a location more precise than GPS (and for when gps doesn’t work); eg ICP point cloud matching/alignment/registration. More sparse maps would give you a less confident match, and you’d have to convert what the car is seeing to the sparse form.