Although we assume that a cat's purr is an expression of pleasure or is a means of communication with its young, perhaps the reasons for purring can be deciphered from the more stressful moments in a cat's life. Cats often purr while under duress, such as during a visit to the veterinarian or when recovering from injury. Thus, not all purring cats appear to be content or pleased with their current circumstances. This riddle has lead researchers to investigate how cats purr, which is also still under debate. Scientists have demonstrated that cats produce the purr through intermittent signaling of the laryngeal and diaphragmatic muscles. Cats purr during both inhalation and exhalation with a consistent pattern and frequency between 25 and 150 Hertz. Various investigators have shown that sound frequencies in this range can improve bone density and promote healing. Cats have adapted to conserve energy via long periods of rest and sleep, it is possible that purring is a low energy mechanism that stimulates muscles and bones without a lot of energy. Perhaps cats' purring helps alleviate the dysplasia or osteoporotic conditions that are more common in their canid cousins. Although it is tempting to state that cats purr because they are happy, it is more plausible that cat purring is a means of communication and a potential source of self-healing.
Short answer: Cats purr as a way to calm themselves, to communicate, self-heal.
