Because you can't get very detailed or specific with a sharpie. You can draw boxes and arrows, and you can label things, but you can't get down to the nitty gritty.
And that's a good thing, because there's no need to focus on the small details until you've figured out the big picture. (Why obsess over the design and color of a website before you even know what the website is about? See?) How can you apply the Sharpie Technique to your work in your studio?