Illustrations are traditionally associated with drawing. For instance, the swedish organization for illustrators and graphic designers is called Svenska tecknare (swedish draftsmen). Being a member of that organization I always felt a bit embarrassed because of my limited skills as a draftsman.
Of course I make sketches with pen on paper, but when it comes to the finished product I don't draw if I'm not compelled to do so. But I do doodle, I do scribble, I do Scrawl. Do you get the picture or do you want me to draw a diagram.