From the video, it is to keep them from disrupting truck traffic (and damaging cargoes), not so much to keep them in, but out of traffic, the trucks, and England. I am sure the French authorities would be happy to let them head south, back to their nation of origin.
Last year the wife and I and her sister, b-in-law, and cousin and his wife took a ferry across the English Channel to France entering at Calais.
What we noticed were mobs of dark-skinned men roaming around the outskirts of Calais for ten miles from the port. Many of them were roaming around the fields outside Calais.
We didn't see any of them on the roadway, but then we were in a chartered bus that went to the lovely French town of Boulogne where we had a nice afternoon.
When we went back to Calais we noticed French troops with automatic rifles patrolling the area near the docks.
I'm not sure where the walls started, but there were walls/tall fences by the roads some distance from the docks.