Why do cats purr?

If you think that your cat's purring means he loves you, you're mostly right. "It is a sign of contentment," according to Anita, "but sometimes it's a form of self-comfort when they feel pain."

Purring occurs by the rapid movement of the cat's voice box muscles along with movement of their diaphragm, as their muscles are moving around 20-30 times per second at around 25-150HZ. It can actually be a form of physical therapy, as it assists in mending bones and general physical healing. Cats also purr as a form of bonding with their mother or kittens - so, really, the sentiment behind the noise depends on the situation. 

Source: https://www.goodhousekeeping.com/