Does it do any good to turn the headlights on for a few seconds (then off) before trying to start the car in subzero weather?
It still can but I’d just try and start the engine without it.
My question is why?
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No…unless you want to use up more power from the battery before you start it? 
Yes.
Anybody who lives in a cold climate, knows this as being a fact.
Tester
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Yup. A battery is a chemical device. Getting that chemical reaction started warms it slightly and helps start the car.
If your vehicle has remote entry ( that usually flashes the turn lights ) then opening the door also turns on interior lights and putting the key in might turn on dash lights seems to make physically turning on head lights unnecessary .