Gameworld referees may require a particular part of plot to be mapped out or restricted and therefore will use barriers as plot devices in game.
As refs we know that it’s sometimes a hard push to imagine you’re stood in front of an evil castle where you have to go through and save the NPC that’s gotten kidnapped again. So we use cellophane and other materials to wrap around trees or poles to map out areas that link to other worlds or underground places. This is to help simulate small enclosed spaces, corridors and rooms that would otherwise be open forest or fields.
We have two examples of this beings used so far, the first being a cellophane shield that was put up around a demon by a Priestess of the Sisterhood to stop it from getting to the players at the Queens days celebration. The second example would be where we used cellophane to map out a tear in another dimension to demonic spiders that required the players to gather quest items at the other end by going through a maze of webs.