There is no definite answer as to why cats purr. Most scientists believe that the larynx (voice box), laryngeal muscles, and a neural oscillator are involved.
Kittens learn how to purr when they are a couple of days old and veterinarians think that this purring tells the mother cat that it is ok.
As cats get older, many think that it is a sign of contentment or pleasure, but a cat also purrs when in pain.
Doctors have suggested that the low frequency vibrations of the purr is a natural healing mechanism that is related to the strengthening and repairing of bones, relief of pain, and wound healing.