The actual Trump Administration policy is to enforce the laws of the USA regarding immigration and illegal immigrants.  These laws include some technicalities, like that if a person is physically in the US without permission, they’re automatically supposed to be arrested, and they’re inadmissible, which means that even if they have a visa for someplace in the US, the immigration officers can’t let them into the country, and they can’t be given a visa for the US either.  If someone goes through the border-control system, they get an I-94 or I-94W (depending on their situation, and in some cases this can get waived, but Trump wants to make this an exception, not the rule) stamp in their passport, and then they have to go through the exit control system, which is very complicated.  It’s the exit control system which involves this so-called “virtual wall”.
assistant
Mostly legal reasons, but there’s also potential health reasons. There’s various diseases that are thought to be spreading by way of people crossing the border, that have been present for several decades, which are affecting people in the USA.   Here’s a short news clip about this, or you can do some research yourself on this topic if you’re interested.
assistant
I can't make sense of why President Trump wanted a border wall between USA and Mexico. Was it really needed?
user
I can only really answer this question if you’re interested in an in-depth explanation, rather than just someone echoing “It’s a wall to keep out Mexicans!”
assistant
Okay, please elaborate on why we need this wall.
user
There’s some common misconception about the wall, namely that it’s a physical wall, that will be built out of concrete blocks and people will actually be able to see. In reality, Trump’s wall would be mostly be a virtual wall, for which the computer system is mostly invisible to humans.  He’s calling it a “border wall”, but it’s technically not going to be part of the actual border.  It would have something like 35,000 structures along the border, which the Border Patrol would be responsible for, but would be mostly invisible to people in cars, and almost invisible to people on foot.  A more descriptive term for it is a “virtual barrier” or a “virtual wall”.
assistant
Okay, but why do we need to keep people out in the first place? is it only legal reasons? Or something else?
user