No perfect answers to the question of purring
While purring cats aren’t exactly difficult to come by, they’re still rather mysterious. We know that the sound is made by vibrating their larynx and diaphragm at once, but beyond that we don’t actually know the nuts and bolts of how it’s done. The other big mystery is exactly why your pet cat, or a big cat like a cheetah, purrs in the first place.
Keep petting me, human
One theory for the purring may seem obvious to anyone with a cat in their lap. A purring cat is usually pretty contented, and so perhaps their purr is a way of letting you know that they’re pleased with your company, and would like the petting to continue. The sound of purrs don’t travel far though, so the range of such communications would be limited to lap-sized distances. The catch is that it’s hard to isolate that motivation, especially since other motivations for purring can exist in the same scenarios.
Another option is that the purrs are simply soothing to the cat. Just a humans laugh and cry for their own emotional needs, purring might be calming and comforting to the cat. In fact, it may go further than that, as the average cat purr is 26 Hrz, a frequency of sound that may help the cats heal and have stronger bones. This would be very helpful to animals that have punctuated moments of high-impact hunting between lots and lots of naps. Thus the purring may help keep their bones strong, even while the cat is lounging in the sun.
The final answer may be that there is no single answer. While science tries to answer questions by isolating variables, biology often serves multiple purposes with the same structures whenever possible. So a millipede’s glowing may also help it avoid predators. You can laugh because you’re nervous or because something’s hilarious. So purring may be a cat’s version of social communication and help them relax and get back on their feet faster. Or it’s just a case of cats domesticating humans, convincing us that it’s really important to keep them well stocked with Fancy Feast.
Source: Why Do Cats Purr? It’s Not Just Because They’re Happy by Danielle Venton, Wired