harder knuckles means harder fist means harder to break means you can punch harder. Btw, its only one small link for making your punch harder.
Methods: Sit in a chair with a bucket of sand at your side. While you watch TV punch the sand repeatedly. Now, put the bucket on the other side of you chair, and punch the sand with your other fist until your arm gets tired too! When your fist get tough enough, heat the sand until it is very hot, and start again. As for me, I make a fist, then I squeeze my fist very tight making it much harder. It takes forearm strength to do this, and you will feel it in your elbow. While your fist is this hard you will not be able to block a punch. Only tighten up your fist just before you throw your punch.
Some people may have brittle hands like Floyd Mayweather they will never punch hard and will always take extra hand wraps. You don't need to do outside improvements on your hands, just get your legs and upper body working together and you will hit like Wilder.
The bones on top of hand support the wrist , hardened knuckles helps absorbing shock retracted by a blow and the wrist is harder break, that in turn creates a more powerful punch .