It’s a long contested question, one that we’re getting closer to being able to answer, but it does remain one of the mysteries of the cat world to some degree.
Newest research supposes that the purr comes from the muscles within the cat’s larynx. As the muscles move, they constrict the glottis and as the cat breathes in and out, the air vibrates and we hear purring.
But why does a cat purr? We’ve long believed that it was a sign of contentment in a cat. And while that is often the case, we now also know that cats purr as a form of communication to other cats, that the purr can be used in times of stress, as an appeasement signal to humans and other animals, in times of pain and in times of intense curiosity. These types of purrs can also have a different tone or pitch. Knowing the difference between those purrs in your cat and when interacting with unknown cats can sometimes help determine how they’re feeling at the time.
Pay attention to your cat when they ask for food; that purr could be different from the one you hear when they’re deep in hunting mode, going after that catnip covered mouse. An injured cat might purr, leaving you to think they’re going to be safe for you to pick up, when really, they’re purring as a way to restore balance to their central nervous system and may not be feeling content at all. The most important thing is that we’re paying attention to the whole cat; body language, vocalizations AND purr to make the right decision about how we interact with them.