It's because of something called cupping.
"Cupping" therapy is performed by taking a hollow container (such as a cup) and causing it to contain lower than usual air pressure, often by filling it with warm air from a candle. It is then placed against the flesh where the air cools, pulling tissue into the container from the pressure difference.
This pulling damages the tissue causing internal bleeding and inflammation. This swelling is believed to promote healing; in a sense it does because the body is reacting to injury.
The practice is "alternative medicine" or more accurately a somewhat harmful superstition and quackery. It stems from the recognition that swelling accompanies wounds healing and then attempting to cause swelling without obvious wounds with the thought it would cause healing for whatever other ailment targeted. But of course we should all know stabbing yourself isn't going to heal your asthma, and bruising your back won't help other problems. If anything it would hurt your ability to cope with other injury.