It is widely accepted that they are great training for any athlete, but how are chin ups good for boxing? The answer is somewhat counterintuitive. If you think about it, when you punch, you are using your "pushing" muscles, whereas chin ups mainly seem to train your "pulling" muscles. Of course, you do need to train your pulling muscles for boxing. When you punch, you have to bring your hand back to your body, something that is accomplished with you pulling muscles. In addition, if you get your gloves locked up, you want to be able to get out of it. But that's not the primary purpose.
Chin ups train many different muscle groups. One of the most important is all of your core muscles. How are core muscles important in boxing? First of all, the stronger your core muscles are, the more protection your body will get from blows. More importantly they give you a good foundation to throw your own punches. Although your skeleton holds you up, it is your muscles and viscera that hold you in place. If you don't have core strength, when you throw a punch, your body will absorb too much of the energy. A strong core means a strong transfer of power, and in boxing, you need that power to pack a punch.
Along those same lines, doing chin ups balances out the power you are building in your pushing muscles. In strength training, you cannot overtrain any of your muscles, because it can cause joint problems. Just like having a good core gives you something to push off of, training these pulling muscles with chin ups will only make your other muscles stronger for boxing.
On top of this, once you are doing ten or more chin ups in a row, you will be training your heart and other muscles to endure the taxing aerobic workout of boxing.