The idea is to not give them a choice whether or not they take the official port of entry, it's to make it the only option.
Currently the border either has a low height fence that can be climbed over, holes that can be crawled under, or no fencing at all. We should install the proposed wall with surveillance.
After crossing the desert there aren't many people that will be able to scale a 30 foot steel fence. The current design can't be dug under or destroyed in a reasonable time.
The sections that have gone up have drastically reduced crossings.
https://www.dhs.gov/news/2018/12/12/walls-work
"For example, when we installed a border wall in the Yuma Sector, we have seen border apprehensions decrease by 90 percent."
"On Sunday, when a violent mob of 1,000 people stormed our Southern border, we found the newly constructed portions of the wall to be very effective. In the area of the breach, a group of people tore a hole in the old landing mat fence constructed decades ago and pushed across the border. U.S. Border Patrol agents who responded to the area ultimately dispersed the crowd, which had become assaultive, and apprehended several individuals. All of the individuals were either apprehended or retreated into Mexico. That evening, the fence was repaired. There were no breaches along the newly constructed border wall areas."