The copper lines or ac are a harder alloy. A line running from the engine will have to flex because an engine moves. In a vehicle these days, it will probably be fine, but when trucks were trucks it was a problem because they were driven a lot of miles. Even into the 60's trucks used soft copper for air as well as oil lines. A softer copper alloy lasted much longer. GMC used about a 6" flex hose between the firewall and the engine to solve the problem. If you put AC tubing in a bender and then soft copper from an automotive shop, you can feel the difference.
