It is difficult in today's technical world to get out of the drawing habit and let the "mouse" do the drawing for you. To counter that, all my initial ideas are done on paper with a pen. I may spend hours scribbling in my sketch book before taking it to the next step on the computer. This is the way that I have been accustom to working out ideas, designs and color layouts.
Also, placing random ideas on paper will allow me to go back to it weeks later and formulate a completed illustration. Everything is dated in the sketchbook so it easy to reference. The filing system in the computer is also user friendly but I have to open several folders to get to what I want. Here is the kicker......sometimes I do not know what I want, I just want to browse. That makes the sketch book ideal.
Over the years I have drawing in every size and shape sketchbook. Some that fit in my pocket, others that are leather bound. Within the two past year, I have just been purchasing the same size hard back sketchbook. Now it looks like a set of encyclopedias in my book case but now I know where to find that idea!