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.

Punching bag= workout & major stress reliever. Try it out sometime when your upset. You'll still have problems, but you'll feel better after.

self confidence and fitness. wear some headgear tho eh?

cardio and stress relief.

Besides what others have said so far; boxing increases testosterone.

Testosterone is good for general health, mental health and staves off dementia and alzheimers.

Chopping wood raises testosterone a lot too.

You will have an effective response to "The Knock-out Game".

Boxing skills are great for stand up. Heavy bags build wrist strength and speed bags build reaction time, timing, and hand eye coordination.

Wrestling is the best universal fighting style.

Late to the game, but here's my two cents. Yes, it's great stress relief and can also be an excellent cardio workout, as mentioned. One thing I haven't seen listed is that punching a bag also strengthens the hands, wrists, arms, etc. when it comes to actually dealing with a punch when you land it.

Punch someone in the face and your hand will hurt. Regular use of a bag toughens you up for that.

I'd highly recommend a Body Opponent Bag BOB (centurymartialarts.com) instead of just a traditional bag. You can learn to more accurately place your shots that way.