Establishing Precedence
I'm not sure what exact train of thought has convinced Trump that building a completely worthless fixed fortification is the hill on which he is willing to die but in his pursuit of building, as George S. Patton once said, "a monument to the stupidity of man," he has decided to declare a state of emergency. In of itself, this declaration would be forgotten in a short period of time due to the irrelevance of any border wall built between the United States and Mexico. However, it would establish a precedence that could be used by future presidents for other pet projects.
Nancy Pelosi is already making noise about how a future Democrat president could use this precedence to enforce gun restrictions:
House Speaker Nancy Pelosi warned Republicans on Thursday that a future Democratic president could declare gun violence a national emergency.
[...]
"If the president can declare an emergency on something he has created as an emergency, an illusion that he wants to convey, just think about what a president with different values can present to the American people," Pelosi said.
"You want to talk about a national emergency? Let's talk about today," Pelosi said, referring to the first anniversary of the school shooting in Parkland, Florida, that left 17 dead on Feb. 14, 2018.
She said the shooting was "another manifestation of the epidemic of gun violence in America."
"That's a national emergency. Why don't you declare that an emergency, Mr. President? I wish you would," she said. "But a Democratic president can do that. A Democratic president can declare emergencies as well."
What's noteworthy here is that Pelosi isn't making any real effort to prevent the current president from declaring a state of emergency to push his pet project forward. Instead she's chomping at the bit to use the same power in the future to push her pet project forward.
It's almost as if every politicians has a list of things they want to do that aren't "legal" (as if that means anything when they're the ones who define what is and isn't legal) and are just waiting for one of their cohorts to establish the precedence that will allow them to go through with it.