If done right, you strengthen all parts of your body. A proper punch is thrown from the legs, hips, shoulders and then finally arms. You have to have good cardio to throw punches for 3 minutes straight. This training keeps you in great shape and you also gain the ability to out fight damn near everyone who doesn't train/know what they're doing. You can get bags that are 200 lbs or more, filled with sand. That is a very hard thing to punch. If you can increase your skills to the point where you can punch 200 lbs of contained sand, and send it flying, good luck to anyone who fights you. Blocking won't help them. Having a fit body and knowing you can drop damn near anybody gives you confidence which spills over into all other areas of your life.