If you were a human trafficker you would prefer “papers please”. Without that law your stock might wander off and find their own freedom. With ICE roaming about the trafficked are more scared to be in public. I’m not sure how people are trafficked in France but out here they do so in secret compartments in trucks.

Your question of enforcement only makes sense if you think “papers please” is the only way to enforce this law. Other means of enforcement include requiring papers when applying for employment or any government program or purchasing interstate travel or purchasing lethal means.

Some problems are good to have.

There is a Big difference between requiring documentation to stay in a country and enforcing said policy by requiring everyone to carry papers.