Imagine you can’t see or hear anything but you can read a compass, and you have an internal map of your house and neighborhood. You also know how long your steps are with some amount of accuracy. You would probably be able to get out of your house and maybe to the corner store, but the inaccuracies in your compass and distance estimation would add up over time, and on a long walk you might overshoot the sidewalk and walk down the middle of a busy street by mistake.