I can see how this would confuse a few people. Something that you actually need to use some judgement on. There are a few ways to look at it. You want to keep things fairly dark for takeoff so you can see outside. If you are worried about electrical failure leave the light on low. For cruise especially on long legs it helps alertness to have the lights bright. Around top od descent start making it darker so that for landing you have your night vision back and can see outside better for landing and taxi. That's the way I always did it on the bus.