I noticed something about DRLs a few years back when I was driving in the (CA) South Bay on a freeway - it was dark - definitely night - and there were a number of cars quite obviously driving "lights out" but with DRLs, and I've seen this a number of times since - though only on well-lit highways.
I suspect that the driver sees that some light is coming from their car, and there are enough street lights that they see well enough, though people coming up behind them don't see them so well... this is my suspicion based on the fact that of all the cars I saw driving without lights turned on (ie, no taillights at all) all had DRLs (which in older cars means low beams on lower than normal setting).