For me on this particular quilt it makes sense. The feathers have an organic form but still have a common spine on all four corners. They will also be trapuntoed. The trapunto means the quilt must be premarked so I know where to sew my water soluble thread and because each spine must match to add unity to the quilt they must also be drawn on exactly how I need them. Because the feathers are organic, each one differs and this can lead to mistakes. If I marked up the quilt once it is finished and I make a mistake all the quilt markings on the whole quilt would need to be washed out. By marking first if I do make a mistake it's only the piece I'm working on which would need to be started again.
I mark using a Berol Handwriting Pen. This gives a nice fine line and the pen doesn't 'drag' the way it does with some other marking methods. It washes out on a normal machine cycle. It does NOT rinse out, but does wash out.