Unfinished paintings reveal insights about an artist's working methods.
Note the careful spacing of the steps and tiles, and the diagonally sloping centerline from the floor through the feet of the priest, to a point on the eye level. I believe this line helped him establish the correct relative heights of the figures.
One art historian wondered why Menzel painted that blank-looking face staring out at us. I believe that's just a stand-in figure placed there for measurement to set the heights of the other figures. That figure would not have appeared in the final work.
Such careful preliminary underdrawing would be necessary for an accurate study like this one, too. In fact, you can see some more uncovered preliminary lines in the lion in the lower right.
Edit: Blog reader Ken did this diagram to explain. Thanks, Ken!