Back when all shotgun shells were paper, most if not all were waxed for waterproofing. After reloading several times, or getting kicked around the truck or shell bag, the mouths would get damaged, raggedy, and out of round making them hard to reload.
The iron was a tapered aluminum rod heating element that was stepped to accept 12, 16, and 20 gauge shells. When plugged in and the iron hot, one would twist the empty shell over the iron where the heat would bring the wax out and make the paper shells like brand new.
You could get a lot more loadings out of shells using one.
