To keep out Mexicans, that is.
For purposes of argument, let us say you are anti-immigration. And let us say the fence would cost nothing to build and maintain. You still might not want one.
Mexicans illegals enter the U.S. through two major channels. They run (or swim) across the border, or they buy illegal papers. Usually the papers cost more than the hiring the crossing guide. The papers make for an easier and safer journey, for obvious reasons. Mexican women, I might add, are more likely to use false papers, given their (their father’s?) greater aversion to the physical strain of four days in the desert.
If you shut off the desert walks (assume the fence is impregnable, ha!), more Mexicans will use illegal papers.
Did I add I would expect the cost of the papers to fall, not rise? Many Mexicans don’t trust the purchase of papers, as opposed to the desert walk. If the walk were impossible, networks for manufacture and sale of the papers would become much better developed. The illegal papers would become much cheaper and much more widely used.
In other words, more young women will come. Many of the Mexican men will have wives here, not back home. Many more young Mexicans will be born on U.S. soil.
Get the picture? Hispanamerica is coming, like it or not. Let’s deal with it constructively.