V max range, why the origin of the graph?

[EngineOut]

I understand that Max range cruise speed is determined from the total drag curve and that it the point where a line drawn from the origin of the graph is tangential to the curve.

My question is, why is the line drawn from the origin of the graph? What is the significance of this point being drawn from the origin and meeting the drag graph at a tangent?

Thanks.

EO