Story and video here (warning: video plays automatically). He also indicated that although the money would initially come from U.S. funds, Mexico would ultimately be reimbursing the United States.
Just as China's Great Wall was constructed "to protect the Chinese states and empires against the raids and invasions of the various nomadic groups of the Eurasian Steppe," this one should help protect against raids and invasions of various nomadic groups of Central America. Of course, a wall isn't enough on its own.
Why should preppers support building a wall? For the same reason that Roman citizens might have been happy with a wall to stop or slow the invasion of Huns, or the Chinese wanted a buffer against raids by the Mongols.