What is the purpose of arrows showing the “direction of the fold”?
“It would seem that if we have a fold line, then it is possible to fold the part, and the direction does not really matter. This is paper, and we can fold it in the opposite direction if needed”.
In fact, it is not that easy. If it were a regular paper (for instance, paper from notebooks or white office paper A4), then it would have been easy to fold parts of polyhedron in any direction without any visible damages.
However, if it is a thick, glossy paper or cardboard, any fold creates damage to the surface. If you fold the paper a couple of times, you will see the damage with the naked eye.
To avoid that problem, we point out the fold's direction and recommend to fold paper only once.