Why was the pattern of alternating reversals of earth's magnetic field as recorded in the sea floor rocks such an important piece of evidence for advancing plate tectonics?

Rocks are formed when magma spews out of the crust at divergent plate boundaries and cools and solidifies when subjected to atmospheric air. The older a rock is, the further it will be from a plate boundary. Moreover, the molten rock contains magnetic components that point in the direction of the magnetic field at that time. Upon solidification, the magnetic materials get "locked" in the orientation they are in.

When rocks at different distances from divergent boundaries were observed to have magnetic materials aligned in different directions, the understanding of plate tectonics as a dynamic process improved. 

Newtons laws of motion