A thick marker makes detail impossible, and that’s the point. When you sketch with something that can’t draw a fine line, you’re forced to stay at the level of structure and flow. The roughness signals to everyone in the room that nothing here is final.
Working by hand introduces resistance. Mistakes can’t be undone, you have to cross out, redraw, reconsider, and that friction is part of the thinking. A rough sketch says this is for thinking, not approval, in a way a polished wireframe never does. Breadboarding over wireframes works from the same constraint, low fidelity keeps the conversation where it’s still useful to question.