My wife was cooking a recipe and was confused by instructions to put foil over a cookie sheet which should sit on a broiling pan. Foil is pretty standard in recipes, but why have the sheet on top of the broiling pan? She was cooking shrimp.
It's possible that the recipe author was going for insulation from the heat of the oven's element/flames on the bottom. This would give you more cooking by convection - or baking without the browning on the bottom (conduction) where the food touched the thin pan.
Another way around this is insulated baking sheets - two layers with air in the middle. On cookies, they take longer and reduce browning. So this seems to me to be an approach to the insulated baking sheet, creating a workaround on the assumption that people have the other equipment at hand.