Its to do with the emergency procedures for the airline and emergency lighting requrements in CAO 20.11
When an aircraft is in flight and less than 1,000 feet above terrain, emergency lighting system shall be switched on, or normal cabin lights shall be on and the emergency lighting system armed in the Flight Deck. Armed means the lights will automatically function when aircraft power is lost.
As the cabin lights are on but dimmed the emergeny lighting in the cabin is armed, rather than the cabin lights being off and possibly alarming a nervous passenger with all the emergency lighting on.
I have only finished my first week of groundschool for a passenger jet so I may be wrong, but that would be my guess.