As well as why cats and why it exclusively privilege cat's, actually anybody precisely doesn't know purr. However in this respect there are some reasons. Consider that purring of cats is result of generating of electric impulses in a head brain which are transferred through the central nervous system, leading to reduction of the certain muscles located about vocal chords, they are reduced and resound. Then vibration is transferred to a body, but heard sounds start with a nasal cavity and a mouth. And why cats purr are already another story altogether

Here some theories: It supports efficiency of blood circulation and, thus, keeps health of a cat. Silent purring is a requirement, and loud - gratitude that an animal has received that wanted, usually food. It is a signal which mother calms the kittens, and at sounds with which kittens let know swear that everything is all right. The dominating cat will purr to show to the subordinate that he isn't going to attack. Cats purr, when are afraid or can be subjected an attack, saying to that that they small and defenseless beings which can't be offended. Patients or wounded cats purr to support themselves and to show to another, what they unfortunate. Purring - the certificate of gratitude, happiness, satisfaction of a cat. The majority of cats will agree with such treatment. Помурлыкай, and your owner, most likely, will give everything that you want, even if it is last shrimp on a table, - cats not such and silly.