Actual boundaries are a physical and psychological deterrent, as well as an area of demarcation that display a willingness to deny access to a zone, usually by some authority. Granted these people clearly do not respect that authority, but the problems of besting a physical object like a wall reinforce the nature of the consequences of disobeying it. The short of what I am saying is that walls do work at keeping people out, especially if it is well noted why it is there and what the punishment is for failing to heed it is once on the other side. No less, it needn't even be one-hundred percent effective to have a positive gain. Not just on immigration issues, but proving the United States can better back its word for once after the past few years.

I live and work in a remote desert. It is not that difficult to build a wall up to a point that is difficult to pass then use the natural boundary to funnel people through it and into another, significantly smaller wall. This type of "channeling" is not new and is, in fact, a very efficient way to force people to go right where you want them to; most are willing to believe there won't be a deterrent or anyone waiting on the other side. It also makes escape by those persons more difficult, meaning apprehension and arrest take less time while also being less dangerous for the officers in question. It needn't even be that elaborate as having anyone on the other side to remain fairly effective.

It was said already, but a physical boundary can be used for anything from solar panels to wind turbine in the right places, but its objective isn't to off set its costs; that's just a benefit that should be worked out if at all possible. The reality of managing the upkeep of a mostly concrete and steel structure is minimal in a desert; it isn't like it is going to rust that much. More Border Patrol agents would be required to do this, but that was a given from the start as the objective was to reduce and eliminate, as near to zero, illegal immigrants gaining unauthorized entry and to do just that you will need human sentries.