Your eyes need to be adjusted to darkness for what may happen inside the cabin. If for example you suffer a problem on take off or landing which requires an evacuation. If the APU (Auxiliary Power Unit) is not running, when the engines are shut down prior to an evacuation, the cabin will be plunged into darkness. For this reason, your eyes need to be adjusted to the dark so you can get yourself to the exit. It does not matter if its dark outside, the fire from the burning aircraft will light your path!
