Here's what purpose.
Mexico is stripped of this land circled in red and it's turned into an illegals drop off.
Mexico willingly gives up this land for this reason because they know they opened the doors to flood the US.
It was wrong they know it was wrong so basically we get a huge chunk of their land we get to do what ever we want with.

I figure the yellow circle can be for beaches resorts ports and maybe naval base

This should have been bart of the BBB
Maybe thext bill