The purpose of breaking boards is to see and experience the ability to create impact force. (A single board provides so much resistance, additional boards, without spacers*, more resistance.)
Force equals mass times acceleration. F=ma
The kicker creates force by the acceleration of their mass and “registers” the force created by the foot impacting the boards. It is important for the kicker not only to create this force but to form the shape of the foot at impact so as to reduce injury to the kicker.
A human does not necessarily stand still to absorb a blow but if the blow, in this case, the kick, were delivered to a human body, a huge amount of force would be absorbed in that body. Much damage, a very potent weapon, and definitely not a toy.
*Spacers create a domino effect, one breaking the next until the force is dissipated in increments. The cause and effect give very little indication of how much force was actually created.