Why Do Cats Purr?

The muscles around the voice box act as a valve by rapid succession of contracting, almost an opening and closing technique, as air is passed through the area on inhalation and exhalation. This rapid action causes the muscles to vibrate and create the sound of purring.
It is well known that cats purr to indicate contentment, but they do however, also purr to show intense anxiety and illness, perhaps to provide a method of self-comfort.
The type of purr depends on the individual cat and the mood it's in. Purring can range from a soft, low and husky rhythmical sound, to a high pitched, trill and excited purr.
A cat purring has been known to be a source of great comfort and a stress-relieving means for owners who take their own comfort from it.