Why do cats purr?

Most people think that cats purr to show pleasure or contentment. Purring is a kind of low continous rattling hum, but it is nothing to do with a cat's real voice, for the vibration frequency is far lower than that of the vocal chords. In fact, a mother cat uses purring to call her kittens to feed. at birth kittens cannot see, hear or smell but they can feel the purring of their mother as a vibratory movement and so come towards it to nurse. Once the kittens are feeding, the mother stops purring.(questions and answers)