Wondering why cats purr? Read this article from Care2:
Excerpted from The Cat Behavior Answer Book (Storey, 2007), by Arden Moore.
The phenomenon of purring has fascinated humans for ages. A lot of research has been conducted to figure out this feline mystique, but no one knows for certain why cats purr, though it is believed to be a voluntary act initiated by the central nervous system. In other words, cats purr on purpose, not just as an instinctive response.
Scientists report that cats produce purring sounds by using the diaphragm to push air back and forth across vibrating nerves in the larynx. Purring occurs in a frequency range between 25 and 150 hertz. At the lower end of the range, that rumbling sound can resemble an idling diesel engine, which has a similar velocity.
All domestic cats and most wild felids are are born with the ability to purr. Cats, from young kittens to senior citizens, purr when they are happy, such as when they are being petted, anticipating dinner, or snuggling on a warm, cozy bed. Mother cats purr when nursing their kittens, and kittens purr when nursing.
But many cats also purr when they are afraid or in pain. That helps explain why females may purr during labor and why some cats purr when they are being examined at a veterinary clinic or when they are recovering form an injury. The purring might serve to reassure or comfort the frightened cat, and some studies suggest that the low-level vibrations of purring physically stimulate feline muscles and bones to keep them healthy and actually hasten the healing process.