Honestly? To give her some credibility on the national stage.
She doesn't have the power to do shit with the border unless he signs off. So, he's telling everyone that she's the point person so that they have to deal with her to get proposals to him. It loops her in and forces the players to engage her. Which could be beneficial politically down the road.
It's optics and politics. But in reality? He still makes all of the ultimate decisions.
Not to belittle her but it's like going to a restaurant and telling the waitress that your kid will order for himself. At the end of the day, you're paying the bill and if you don't approve of what your kid is trying to order, you're going to shut it down. The illusion of power.