Capture the Flag - Why hollow box for Barrier shape?

Hi,

I’m following this tutorial: 2. Create the Island

It’s says to put Barrier devices around the map. I understand that as you want to keep the players from escaping. I just don’t get why it would be hollow? Why not solid?

It says:
Zone Shape - Hollow Box - Players will spawn inside the barrier this way.

How would they ever be able to spawn in there?

Maybe I’m setting my map up wrong. I have set my barriers up around the edge of the map like 4 walls. Am I doing it wrong?