During take off or landing, in case the aircraft faces some technical difficulties and has to make an emergency landing, you would have only about 90 secs to evacuate after the plane touches and stops on the ground.
These 90 seconds are crucial to see around, and understand the situation around you probably to see what gates are being used for an evacuation and so on.
The human eye can function from very dark to very bright levels of light; its sensing capabilities reach across nine orders of magnitude. This means that the brightest and the darkest light signal that the eye can sense are a factor of roughly 1,000,000,000 apart.
However, in any given moment of time, the eye can only sense a contrast ratio of 1,000.
Hence your eye needs to adjust to the light for you to actually be able to see anything.
So if the light outside the aircraft is way brighter than the lights in the cabin or its night outside, the cabin lights were too bright and you were suddenly plunged into the darkness and had only 90 seconds to disembark from the aircraft, your eye wouldn’t be able to adapt to the lighting condition of your surroundings.
This exactly why the cabin lights are also dimmed while landing at night! Passengers are also asked to fold up their tables and straighten their seats during take-off and landing in the case of an emergency to ensure a quick exit.
Check out this cool video showing the eye adjusting to light.