My understanding is that they are death traps designed to pull you under into a net and then you drown. There are warning signs on the US side but not the Mexican one.
Not sure how they work as a deterrent, maybe the goal is to kill enough people so news goes back to the migrants’ countries and scares the others off the trip? Otherwise if you already made it all the way to the border I don’t think you’d turn back because of them. Then again maybe the point is simply to kill migrants.