I use a headlight during the day on flashing mode to be seen. My commute to work has a stretch down Penn avenue where I maintain 30 mph and it is imperative that cars turning out into the road can see me from a distance.
I use a headlight at night on steady mode to be seen and to see the road ahead of me. If I'm coming home late from work, there are some especially dark stretches through the strip with potholes that will eat a bike wheel.