Think of yourself on a boat out at sea on an overcast day when you can't see the sun or the coast or even the water beneath you. However, if there is something there that you know is not moving, such as a lighthouse or island, then you can figure out the direction and speed of yourself and then figure out the directions and speeds of everybody else.
Hotspots are relavent to the plate tectonic model in that way.
They are fixed points on the Earth. They don't move relative to the Earth or relative to eachother and they therefore provide reference points from which to determine the direction and speeds that each plate is moving at. All of those possibilities would give you the same result. The hotspot has given you a frame of refernce, and from that you can figure out all the other motions. I hope this long-winded answer has helped to explain your question.