I have to agree that it looks like part of a 2 or 3-piece chaffing dish - probably with a metal, ceramic, Pyrex, or Corningware insert (2-pc) which may have a lid (making it 3-pc). I have one similar piece but it's not as fancy - it's a ceramic baking dish with a basket holder.
As for idea of it being good to hold in the heat because copper is a good heat conductor ... the opposite is true. A good heat conductor, like copper, heats up quickly - and cools off just as quickly. If it was intended to retain heat - it would have been made of a poor heat conductor, like cast iron or straw.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain