I've worked for a couple of UK carter airlines now and both gave the same reasons during training.
Firstly so the all occupants of the cabin (crew and pax alike) can see outside and to be able to report anything untoward - or check for fire before opening doors etc
Secondly, so that everyone's eyes become accustomed to the light outside (also the reason that cabin lights are dimmed during take-off and landing in the hours of darkness) should there be a need for an evacuation.
Markie