I think Finski introduced the heater idea a couple years ago. The weather here is very unsettled in the spring, 89 on Sunday, chance of snow this Friday. So I tried it with 10 packages a couple years ago. They did great, so I used it with some small splits last year as well.
Cold doesn't kill bees. They may cold starve, but that's a different problem. I think a lot of people experienced winter losses this year because it was too warm, the bees kept brooding up, and used up their stores when normally they would have been clustered. The heater is to help them to expand the brood nest and build up faster. It has been discussed at length so look for threads on terrarium heaters.