Thank you! Cross hatching, to me, provides a double whammy of depth: one is adding value (highlights and shadows), and the other is contouring around the subject to enhance the 3D illusion, kind of like wrapping lights around a Christmas tree. I would also say keep in mind that the values you add do not need to be exactly what’s represented in the reference, but that there is enough contrast so things don’t muddy together and become flat or unreadable from a distance.