Is that it? Just a bunch of crates that ''god'' drops before the battle? meh...
Well no, it's generally explained in the narrative as you having the time to prepare (Which is why it's called the preparation phase), this means you get to place the barricades where you think it makes sense. You can bring in a certain number of barricades and traps and place them during the preparition phase. During combat, if they get destroyed or your traps are sprung, they're spent and you'll have to construct new ones when you make camp.