Why do they dim the cabin lights for take off and landing

Why do they dim the cabin lights for take off and landing?  Ask a stewardess and they will tell you it is FAA regulations or some other hogwash, the fact is they have not got a clue

And it turns out no one else really understands why it is done save that it is a throwback form the times when in a small plane it was important for the pilot not to be distracted by cabin lights if and when the door twist cockpit and cabin was opened at night

Fact is the cabin door is now never opened and stays bolted firmly closed to protect the flight crew from the passengers they are paid handsomely to transport around the globe