@Copperpipe -- Devil's claw is a shrub from Africa that is also used medicinally.
Many use devil's claw as an alternative painkiller, and it has been linked with reducing arthritis and other joint problems.
It's also common to use devil's claw as an anti-inflammatory, and to reduce back and neck pain.
As for why it's combined with yucca, just look at the benefits of yucca -- it can reduce the symptoms of arthritis, bursitis, and joint pain. It's better to have two herbs doing the same thing, I guess.
I do know that devil's claw and yucca is used for dogs and horses as well, but I'm not sure about the benefits for animals.
You may want to ask a holistic health practitioner for more information -- and don't forget to tell your doctor you're thinking about taking a supplement -- yucca can interact with certain drugs, so be sure to run it by somebody in the know first.