Much like lifting weights, punching stuff will make your fists stronger. The harder the surface the better the effects as with each cycle of healing, the bones in your fists will become denser. Your fists will adapt to the sudden impacts and eventually you'll have a super strong and deadly punch.
Now this is all based on 100% truths and unless you can prove the opposite with a living specimen, it'll stay that way.
There's even a guy who trained his finger to the extreme. He is even able to pierce coconuts with a single finger. His finger was broken numerous times during his training and definitely appears crooked but it is without a doubt one of the strongest fingers out there.
So get out there and break your bones, just make sure to let them heal properly or risk a crooked finger.