Trump's Wall is nothing more than a big fat dogwhistle. Like Australia's offshore detention.
Trump is trying to appeal to his base of stupid voters by making a big gesture showing how he is protecting them from the scary foreigners.
The GOP's real support base, of course, is billionaires and big corporations. And they LOVE immigration. What Trump (like Dutton, Abbott and co in Australia) is doing is serving the billionaires and big corporations as he must, while making the precariat believe that he is taking care of their interests.