So I understand how to use Mohrs circle and the transformation equations to find principal stresses and stresses for a given plane, but what is the point? Is there a purpose to knowing this other than finding stresses for a given direction, the stress invariants or that shear does not occur on the principal directions? How are these things used in the real world / design? Don't people usually use average stress when designing? I guess I just don't see a practical application.