I love having a cat settle in my lap, get comfortable, and start to purr. The low rumbling sound is relaxing for the both of us, and helps me de-stress at the end of the day. What is purring and why do cats do it?
Purring happens when the brain sends messages to the laryngeal muscles causing them to vibrate when the cat breathes in and out. Sounds simple, but scientists don't understand how these messages are generated and controlled. Purr studies have been done to try to figure it out.
So why do cats do it? One theory is that purring is a form of communication, relating to others the cat's pleasure or calmness. This could be true, but it is also known that cats will purr when they are injured or frightened. Another theory is that purring is self-soothing for the cat, which would explain why she would use it in stressful situations. As I was searching for answers, I found something else:
Cats may purr for self healing. It is known that cats purr at a frequency between 25 and 150 Hertz. Apparently vibrations in this range are therapeutic for bone, muscle and ligament injuries and pain relief. It is also theorized that since cats are inactive most of the day to conserve energy, bone density loss and muscle atrophy could occur. Purring may prevent that. The Felid Purr: A bio-mechanical healing mechanism article discusses this.
I believe that all of the theories have some merit. My cat is multi-tasking. She is curled up comfortably on my warm lap, meditating, conserving energy before the next hunt (well, dinner served on a plate), keeping her bones and muscles ready to leap to the top of the cat tree to get the best spot.