On landing/take off at night they will normally be turned down to the lowest setting to ensure rapid acclimatisation to outside lighting conditions in the event of an evactuation.
That reason has been shown to be a myth. Much was written on this subject in Pprune pages going back several years. Passengers are permitted to use overhead reading lights without restriction and this immediately nullifies supposed night adaption of the eyes of not only the passengers in the immediate vicinity but the cabin attendants who have no choice but to see the lights. The cockpit crew have no night adaption on taxi, take off and landing simply because of the bright landing lights and the bright runway lights. The problem with switching the cabin lights to dim is that elderly passengers and those who wear glasses are less able to read the emergency card instructions or see the emergency exit signs properly. Finally there is no restriction on cabin staff having the galley lights on bright during take off and landing and that means their night adaption is lost. Far safer for all cabin lights on bright throughout the take off and landing phase so that exit signs and doors are easily seen and their position remembered if all the lights go out during an aborted take off. Some airlines leave all cabin lights on bright while others go for the dim option. This suggests that opinions differ on the night adaption efficiency question. Medical evidence reveals at least 30 minutes of total darkness is needed for true night adaption. In which case it is next to useless to have the cabin lights dimmed simply because of a possible evacuation. And of course, any fire outside the aircraft would also ruin night adaptiopn.