A point (called a tang) is cut at one end of the rectangle for feeding through the drawplate and providing something to grab onto. The first few holes are easy to get through and the sheet is forced from a U shape into more of a tube. As the opposite sides of the rectangle touch, it becomes more and more difficult to draw down into a nice neat tube.
The finished result is a length of tube that can be soldered down it's (inevitably) curved seam topped by a mangled centimeter or two of material that is unusable. This all seems terribly wasteful to me, and the tube we produced did not even have a nice round centred hole down the middle.