Why do Cats Purr?

Cats purr for numerous reasons, although the most commonly known reason is a sign of contentment. When Fluffy is sitting in your lap, it is only obvious that the vibrations you're feeling are due to happiness. However, purring can signify a problem.

Cats also purr to heal wounds. Excessive purring may be a warning sign of an injury. Quoted: "It seems that the measurable Hertz of a cat's purr lies between 25 and 150. Coincidentally (or not) it has been found that sound frequency in this range can stimilate bone growth and healing." ¹

¹http://cats.about.com/cs/catmanagement101/a/why_cats_purr.htm