Its no ploy.
Dachshunds need extra vitamins to support there back since they are known for having back problems later in life, if you look up the breed specifics on eukanuba or royal canin, they list what special ingredients are in there food.
Theres a huge difference between old roy and the higherbrands.
www.hotdogblog.com has a list of what exactly these foods do to benefit the dogs.