Why do cats purr?

Cats purr using the laryngeal and diaphragmatic muscles. They purr with a frequency of between 25 and 150 Hertz, a frequency range which, according to investigations, promotes healing and higher bone density. Although a cat’s purr is often associated with contentment, many cats purr during mores stressful times, such as during visits to the vet. Thus, it is unclear exactly when or how cats purr, prompting further research into this phenomenon.
Many cats purr when injured or ill. Some posit that it might be a mode of self-soothing. But, other research shows that the vibration of their body could actually help heal wounds, alleviate swelling, and build muscle.  In fact, the association between purring and the healing of bones and muscles could have applications for space travel: bone loss and muscle atrophy are a concern for those in zero gravity.
It is also posited that because cats have adapted to conserve energy by sleeping for long periods of time, purring might be a way to stimulate the body without using much energy, alleviating dysplasia and other conditions often seen in dogs.
Works cited: Lyons, Leslie. “Why do cats purr?” Scientific American. April 3 2006. Web. 27 August 2015.
“Why do cats purr?” WebMD. Web. 27 August 2015.