In addition to what has already been said
Maybe you're a good puncher but sometimes the other guy is better with his head movement and you have a hard time hitting his face (maybe he counters you every time you try, maybe he just dodges your attacks too consistently), so you punch something that isn't his head because you know it will probably land and disrupt him. If the guy ducks a lot then he'll often duck his face right into the punch if you aim at the chest. That's one reason to do it.
Getting hit anywhere in the body, especially repeatedly, takes the steam out of the other guy's own punches because it hurts to punch when part of the body is injured. Punching (correctly) operates on a kinetic chain like a whip, so if you damage a part of it anywhere along the line (including the muscle, bones, joints, nerves, on a person) then that weakens the overall effect down the line because energy bleeds out there-- the damaged link, instead of transmitting the energy onward smoothly and efficiently like it should.
^^That's the mechanical portion of the equation. When a person feels their body send a recurrent pain signal that increases every time they move, their brain can't help but try to slow it down more and more as a deterrent to increasing the wear to that damaged area, regardless of what they decide they want to do, it is overridden by the automatic, non-voluntary portion of their brain.
When a person consciously feels the pain and stiffness and fatigue and notices themselves slowing down, it has a strong psychological effect which compounds the problem even worse.
It gets worse not only with repetition, but also with time. It feels worse/more exausting and the inflammation response gets a person's body to stiffen up (the opposite of what you want to do when boxing or fighting in general). It has a real, tangible cost to the opponent with significantly reduced risk to the one punching.