Is it to work out all that aggression built up from so many bread failures? Not exactly. Punching down the dough releases any gas bubbles that have formed during rising, and also redistributes the yeast, sugar, and moisture within the dough.
To “punch” the dough properly, simply firmly, but gently push your fist into the center of the dough. Then knead the dough a few times and reform the dough into a ball. Shape dough as desired and let rise again (proof) before baking.