Ah, the number of times this has come up!
I've heard:
1. Adjust pax eyes to night in case of emergencies. Doesn't fly, it takes far too long to adapt eyes to darkness for this to make any difference.
2. Make aircraft more visible to others. Well, maybe. Darn thing's got enough lights and strobes, though, and it's not as if between ATC, radar, TCAS and the bods up front who really should know what an a/c looks like, people should be relying on being able to see in to what 36A's up to. And then what about dousing the cabin lights but letting you put your reading light on?
3. Saves seconds if something's going on and somebody needs to look out to find out what. Again, well, maybe. Can't hurt. Can't make much of a difference.
4. In the event, makes it easy for rescue to see where's safest to cut in through the fuselage without hitting the SLF (I can see that if you survived a crash, only to be dismembered by Fireman Sam's big round chopper, this may be considered unlucky). This is the only reason that makes much sense to me, but then I am rather ghoulish.
Has there ever been an incident where this actually would have made a difference? When did this start?
R