Compare the shapes of the bones of the human skull with shape of the bones of the human leg. why is their shape important?

The human skull is one of the thinnest bones in the human body and is not meant for supporting very much weight or for absorbing shock like a human femur, tibula, or fibula.

 the rounded shape of the skull is meant to hold a brain while the cylindrical shape of a "leg bone" is made for support and shock absorption.