Why the angle between Two Vectors in a plane or in a space is not measured from head to tail?

Question

Let us assume two vector a and b such that they lie on the same plane. Then if we want to measure the angle between a and b, For now, let us assume it to be θ Then why is the angle measured from the same initial point although the point of origin doesn't matter ,**why the same starting point?**Further, Why isn't it measured from head to tail as in the the end of vector a would be the origin or Starting point for vector b? What would be the intuitive way to understand it?