Just for the cameras. Just Kabuki Theater..
In many places they stretch out a hundred yards of Concertina wire, then instruct the invaders to go around.
In other places where they stretch a thousand or so meters out, the invaders throw blankets, coats, boards, whatever is avaiilable over it and climb past. Some of the invaders get in such a hurry that they slip off the padding and into the wire. But never fear, there are troops, Cops and Border Patrol there with wirecutters to cut them out and get them on ambulances to take them to nearby US hospitals.
And you know this...how?