While there are some infinitely more knowledgeable people hereabouts than me, here's a stab. An IRS platform needs earth rotation to align itself. Basically, when started its accelerometers measure earth rate to determine it westward drift. From drift it can find East and, hence, 90 degrees to the left is True North. Also, it can approximate its latitude. One application for earth rotation. More simply, Kershner was probably just trying to acquaint his readers with simple navigation and the basics are the arena--the Earth.
GF