It becomes more apparent when you see his videos on the subject from the 1980s - he was planning this move for decades.
When did Mexico agree to pay for the wall?
Plus he's a construction magnate -- get enough traction to get a preliminary budget and start allocating funds to his buddies. Even if it's just a concrete wall for 1/3 of Texas that's big $$$ for his construction cronies.
He's like a vacuum cleaner salesman who can always get his foot in the door, but can never manage to sell a unit.
He's like a vacuum salesman who promises to give his customers dishwashers and toasters for free if they buy a vacuum cleaner.
When they buy the vacuum cleaner, it turns out he lied about the other goods but they still ended up with the vacuum cleaner. But what a lot of people really wanted was the dishwasher.
I may have pushed the analogy a little too far...
Check Scott Adams’ writing on Trump’s persuasion abilities.