Most flashlights are made of aluminum, which is lightweight, strong, and pretty good at conducting heat. For LED flashlights it is important for heat to be carried away from the LED so that it doesn’t overheat and possibly burn. Copper is fantastic at conducting heat, twice as effective as aluminum. However, it also weighs about 3 times as much as aluminum, and is only about one third as strong. It is also typically more expensive.
LED’s are much more efficient than incandescent light bulbs, but still most of the energy they use goes towards heat instead of light. And as you drive the LED harder, it becomes less and less efficient, producing more and more heat. One of my first uses of copper was to wrap copper strips around a P60 drop-in in the head of flashlight, filling up an air gap (air transmits almost no heat) and allowing heat from the drop-in to sink into the head and body of the flashlight. I used aluminum strips at first, cutting 1″x3″ strips from aluminum cans, but eventually I bought some sheets of copper and cut that into strips. Here’s a picture of copper colored aluminum light with the drop-in wrapped in a copper strip.