Cats purr for a variety of reasons, but the most common purpose is to communicate contentment and pleasure. Cats may purr when they are happy or relaxed, or when they are seeking comfort and reassurance. Purring can also be a way for cats to bond with their owners, as it is a sound that humans find pleasing and comforting. Purring can also be a way for cats to self-soothe, as the vibrations of the purr can help to reduce stress and anxiety.