Donald Trump revealed his ideas about building a wall along the Mexican border during an interview with the NYT during his primary campaign. His comments on building the wall are classic Trump. That is, a set of contradictory ideas not connected into a clear idea about the need for a wall. He revealed himself at one point in the interview when he stated that sometimes he lost his audience during his speeches and he could bring the audience back by screaming that he would build the wall. (This was probably right up there with "lock her up")
There are many issues about building the wall and whether it will really do anything about illegal entry into the US from South of the border. It certainly will do nothing to stop the flow of illegal drugs into the US. So why is Trump still planning to build a wall? You guessed it. It is just another one of the campaign promises that he made. Now he has to do something about it in order to hang onto his base. It will cost taxpayers billions so that Trump can look good to his base. In the meantime it has totally soured our relationship with Mexico. They will not pay for the wall in order to satisfy another of Trump's campaign promises.