Welding Related.

Copper conducts heat very well, cooling the weld soon and minimizing heat distortion.

Copper is often used as a temporary backing while filling holes or gaps.

http://weldingweb.com/archive/index.php/t-18181.html

Most welding spatter will not stick to copper. If you are filling a gap, your molten filler metal will stop at the copper and not run through. When you turn the item over to weld the other side, you have a nice flat surface to grind a groove in for a complete weld.

A backing plate of similar metal can get welded down, getting it off to finish the weld can be a lot of work.