Drawing damaged leather using charcoal
Damage to the leather is just like any other detail. It is created by relief (changes in height) in the surface, so it is rendered the same. Just like the embossed areas of the leather, one side of the impression will capture light and the other will cast shadow.
So to draw scratches etc., you would use light edges and dark edges along the contour of the scratch.
As far as specifically using charcoal versus pencil that is a stylistic choice. If the charcoal medium does not deliver the look you want, then you would probably want to stick with pencil.