A cat purr is a comforting sound. But what does it actually mean?
Cats purr when they feel comfortable, that much is known. Unfortunately, for the hypothesis that cats only purr when happy, they also purr when they are hurt or scared.
Cats purr by vibrating their larynx and diaphragm. Apparently, purring stimulates bone growth in cats, since vibration helps retain bone density.
Cats may be purring as a form of communication, letting what is close by know that they are comfortable with them being that near. Cats in the wild will often lay near an injured family member and purr either to comfort the other or promote healing vibrations.
Purring could also be a way for the cat to calm itself down. It might be a self-comforting act that could release a reaction in the brain to make it feel less pain.
Unfortunately the jury is still out as to the singular reason cats purr.
But in the end, do biological answers really matter when you have a purring cat in your lap and you know they are content?