I think its usually so the passengers eyes can adapt to the low light so in the event of an emergency or accident and an evacuation is nescessary you can see your way to the nearest exit more easily because the cabin at best would be very dim at night if the aircraft had sustained damage in an over run or something similar. Although not all airlines have the policy some have the lights on max for takeoff and landing, reason being rescue services can see the aircraft more easily if an accident occurs, hmm