Ever noticed the sign on the highway advising you to turn on your headlights thru the tunnel?
This is not necessarily so your lights may illuminate the road for you, perhaps more so that vehicles behind you can still see you, especially if they are still driving with their sunglasses on (take them off!).
If the traffic suddenly stops for some reason, do you want the vehicle behind to rear-end you, or do you want him/her to see you clearly? Turn on your headlights all the way; this ensures your rear lights are also on.