The welding industry calls that a "backing bar". It is definately not ad hype. Some backing bars, for certain welding applications, are made to smother the back of the weld with inert gas to protect the back of the weld. When you are MIG welding you have a shielding gas protecting the weld pool from contamination. If you have 100% penetration, then the back of the weld is exposed to the atmosphere and is contaminated. Anything you can put on the back, can only help, even if just a little.
