dimming the lights

I realise that the extra hour today has left me with too much time to think ... but on a night flight they always dim the lights for take off but you are free to put your reading light on so sort of defeats the point - whatever the point is.

I also wonder why airlines are so obsessed with opening and closing the window blinds for no apparent reason. I remember when I flew UA as soon as we were airborne they wanted to close the blinds even on a daytime flight.

These things bother me more than they ought.

The lights are dimmed for take off and landing so your eyes are adjusted to the darkness should there be a sudden need for an evacuation

It may be a myth, but I always thought the reason the window blinds had to be open for takeoff and landing was so that, in the event of an accident, the emergency services would be able to see in to the windows to assess the situation inside the aircraft...

both of these answers make perfect sense - thanks