It's a very odd thing for me -- sketching as planning. I tend to like to work improvisationally, so I dislike planning ahead. When I made quilts, I avoided sketching and simply started directly with fabric. Why do I find myself really called to sketch beads and silver work??
Maybe the challenges of the media are great enough that having a plan keeps me from losing track of where I'm headed. Maybe the speed with which I can execute the work keeps me from getting bored, even if I do have a plan. Maybe the sketches are more faithful for glass and metal than for fabric, so I don't deviate so much from a plan.
Maybe...I am learning to love the sketching for its own sake.