Despite the dimmed cabin lights passengers were permitted to have their overhead reading lights on which of course permitted them to read the emergency card in the back of the seats but which neatly destroyed any night vision adaption for passenger evacuation.
Dimming lights eliminates reflections so it is easier to see out the window.
In daylight hours it makes very little difference.
I've written before that policies and procedures are not factoring the prolific use of cellphones and tablets.
Passengers are pointing cellphone flashlights into each other's eyes during evacuations because the cabin lights are dimmed.
Mjb