As designed for cafes, the doser chamber is filled such that the wedges formed by the vanes meter out a consistent volume of coffee for a single espresso. One pull = single espresso, two pulls = double espresso. Your photo shows one of the doser vanes with an arm perpendicular to the doser bottom. Its purpose is to dislodge / mix in coffee that would otherwise accumulate around the perimeter of the doser. In a home environment where baristas advance the doser arm until the chamber is empty, it serves no purpose.