Previous comments noted, but it's my observation that even with a significant number of passengers with their reading lights ON, the overall light in the cabin is MUCH less when the main cabin lighting is turned OFF.
It's nothing to do with adaption to night vision, it's all about light gradient, that is, vision from light to dark (as during an evacuation) is much improved as the light at the viewer's end is decreased, whether they be night adapted or not. Ascertaining the safest escape route is the reason.
It's for similar reasons that the window shades must be UP for Takeoff and Landing.
Regards,
Old Smokey