I will admit to being a geek, I used a pocket protector through most of the 1980s-1990s. During that time a good portion of my work was doing a variety of tasks involving drawing with pen and/or pencil on paper and/or mylar. That requires a variety of instruments - eg - at the minimum - black pens of multiple widths, red pen, mechanical pencil, plus often another colored pen, and possibly "invisible" pencils (purple for marking tracings to be blueprinted, yellow for paper to be photocopied).
Carrying your basic drawing kit in your shirt pocket is extremely handy.