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When a plane is flying, it needs a lot of speed to keep producing lift. However, when it wants to land, it can't simply brake in middair and stop flying - if it turned off all its engines, the plane would crash. Instead the pilot needs to land steadily, which means he'll still be going quite fast when he hits the runway. He'll need the entire length to slow down, hitting the brakes like you would on a car. The stop isn't instant, which is why he needs a distance to do so.