This is a practice that is very common, but seems ridiculous to me. In my opinion, flares should be a night-time warning device only. The flares are not that visible in the daytime, and cones would be a much better choice. The only possible reason I could see to use flares over cones is that you can fit more of them in a police cruiser.
Even more ridiculous is railroad crews lighting them to stop traffic for a train. There is no reason not to use a STOP paddle or flag.
This picture on Railpictures is a good example. They are pulling train cars out of storage on an abandoned line, and have to pass through a signalized grossing where the signals are no functional.
http://www.railpictures.net/viewphoto.php?id=419162&nseq=2 (http://www.railpictures.net/viewphoto.php?id=419162&nseq=2)
Also, flares in my opinion, are an archaic warning device, that, thanks to modern technology is no longer needed. I think "LED Flares" should be used instead. They don't have to be lit, they are re-usable, they don't stink, and they don't leave that ceramic-like crap behind on the road.
So, why are they still being used? Are people so stuck in their old ways that they can't see the new, better way?
Old style flares (or fusees as some railroads call them) still have application. For one thing they can be left unattended, and will self-extinguish when they burn down completely. And second, they are still very bright and effective at night, though I agree that on a bright sunny day cones are more effective.
Because I'm kind of old like road flares, they still say "Stay the &$£#% away!" to me more emphatically than a reflector or light.
I think I started a conversation here about smudge pots once. Those seem archaic to me. Flares are current by comparison. But they are probably as uncommon now as smudge pots were when I was a kid.
We still use fusees when one of our crossings have malfunctioning signals, but we won't burn one unless darkness has fallen. Every now and then, a railroad official will perform an efficiency test for engineers by lighting a fusee and checking to see if the engineer will stop properly.
I saw WashDOT using flares the other day along SR 99 to indicate a lane closure. It was sunny and, personally, they were very easy to see. I still prefer cones, however.
When the church I go to lets out they usually have a DE state cop directing traffic on SR 896 so as to give the hoard a chance to exit safely. I've seen cops using flares there on several occaisions in varying weather conditions.