Lights are especially valuable early in the morning and late in the afternoon when the sun is low. Drivers looking thru a dirty windshield into the sun really can't see cyclists very well. And if a rider is in a shadow or under a bridge, as a driver moves from bright light into a darker area their eyes do not adapt quickly enough to the lower levels of light and it is easy to not see a rider.

Last year I was stopped at a light on a road heading west about 30 min before sunset. A driver pulled up beside me and told me he was able to see my flashing taillight despite the sun straight ahead. I have a Cygolite Hotshot, btw.

I'm probably going to add a white flasher on the front. I've had several instances of drivers turning left in front of me or pulling out of a driveway on my right. No crazy close calls yet, and I don't know for sure that it was because they didn't see me, but it can't hurt. The flashing itself helps attract a driver's attention in my experience, because it is out of the ordinary. If my flashing light catches the attention of a driver glancing up from his smartphone, hopefully that makes me safer.