Why Do Cats Purr?

I suppose any cat owner can answer this to a degree. It is one part of cat behavior that is easy to recognize and interpret. But there is a little bit more to it than just a sign of happiness or contentment. Why do some cats purr more than others? Why do cats sometimes purr when they are quite clearly not happy? Interestingly, a cat will often purr when giving birth -- is this for their own benefit or that of their kittens?
Most humans like the noise of a cat purring, it is therapeutic. But does it make us feel good because it is confirmation that our cat is happy or is there another explanation?
Obviously cats purr for a reason and it has often been suggested that it actually does the cat good to purr. We don't purr so we don't really know. There are, however, things that we do when we feel contentment that benefit us too. When we feel relaxed we often do things to heighten the feeling. Of course the cat doesn't have the opportunity to put on some relaxing music or dim the lights and light some candles. Could they have evolved a way to de-stress themselves, just by making a noise?
Research has shown that noises in the frequency range of a cat's purr help to heal bones and heal the body; these cat's aren't stupid, but of course you knew that. This could explain why cats purr when under stress of in danger. Is it possible that their purr could calm an attacker? Why not, it calms us. When humans are in stressful situations we often take steps to de-stress; cue the music and candles again. Singing is known to be therapeutic and calming for us humans as is deep and controlled breathing. It wouldn't be any surprise to learn that kitty has developed a similar strategy.
In common with most behaviors in all animals, not just cats and humans, there is an evolutionary aspect too. Any animal that can cope with stress is likely to be naturally selected to further the species. As far as domestic cats are concerned, they certainly benefit from human assistance and caring. If there are things that they can do to attract that, then again, evolution will dictate that that behavior becomes entrenched.
So, next time your kitty curls up on your lap and purrs, remember that it is probably doing the both of you some good.