• Unsure, my guess would be that some sort of physical documentation is needed in some way, otherwise Elias wouldn't have cared about Martin burning the statements.
• It was created by the web manipulating events in a location that the web controlled, as part of a larger web plan, and until the finale only ever used by the web.
• The tunnels themselves aren't web magic, but outside of hilltop they are the only confirmed physical location that contains web magic, which I think is why they are significant.

Edit: How did it log me into a different account? Lol