Scientists have demonstrated that cats produce the purr through intermittent signaling of the laryngeal and diaphragmatic muscles. Cats purr during both inhalation and exhalation with a consistent pattern and frequency between 25 and 150 Hertz. Various investigators have shown that sound frequencies in this range can improve bone density and promote healing.
This association between the frequencies of cats' purrs and improved healing of bones and muscles may provide help for some humans.
quote taken from the Scientific American website, article entitled: Why do cats purr?
Also, you'll never see a cat use a lame set of stairs to get on and off the couch.
I have nothing against dogs, or those who love them. It's just that unless you are looking for drugs, or skiers lost in a mountain, or are a hunter, cats just make more sense (and are more usefull - and in my opinion more enjoyable - to have around).
So take that dog lovers! Cats are just cooler.