Why Does Your Cat Purr?
Purring occurs as a result of vibration of vocal cords due to neurological stimulation from brain activity. The purpose is uncertain but it does seem to be associated with pleasurable activity.
However, cats are also known to purr when ill or injured, which lead some to believe that the frequency of the vibration can be associated with greater healing. Purring also is reinforcing for people and therefore can increase the amount of petting.
Cats generally purr when in contact with someone; a favored owner stroking, nursing a kitten, or greeting a familiar partner-cat. Positive experiences also elicit purring, rolling or rubbing, being in a warm familiar environment or about to fall peacefully asleep.
Thanks: http://sandroadanimals.com/Data/clientservices/eventsatsandroad2/top10mostgoogledqu/DrGoogle-handoutCats.pdf