I just had a realisation. This easel may be intended mostly for painting, where you are holding a brush in a 'writing' position, daubing directly in small movements with your horizontal hand targeted at small areas, which wouldn't be limited by that ledge the way vertically held charcoal pencils or sweeping pencil arcs would be.

Also more than that - that ledge/rest looks designed to hold the thickness of a canvas. The edges of which would be flush with each other or less excess would be protruding, and so you would not catch your hand on it (or as much).

Boom. I knew I was missing something obvious.