Its so your eyes are adjusted to light conditions in case of emergency and lights are cutout. The first few seconds required for your eyes to get adjusted to the darkness, are the most vital in life saving. So the lights in the cabin are dimmed in order for your eyes to be adjusted to darkness.

Another secondary aspect is the self orientation of direction. When the lights are dimmed and the window shutters are drawn up, self orientation as to the direction of the front and rear of the plane remains in your mind in case of emergency evacuation and presence of smoke or very low visibility in the cabin.

One would be very surprised as to how the brain functions in case of panic..

Imagine going from a brightly lit cabin to the ground where its dark in a few seconds down an escape slide?

In situations like this, every moment counts, and airlines try to bank on passengers cooping with the situation. One blinded person can delay a hundred passengers behind him.

Ashraf Raffa