The President can be the face of the nation, bringing encouragement to the people affected by the floods and to the rescue teams and volunteers. In reality, he can do little personally but as the nation's leader his presence is very important.
Last year when Louisiana had all of that terrible flooding, and before President Trump was even elected, he and VP Mike Pence went to Louisiana, and were working along with Franklin Graham, handing out food and water to the refugees and the rescue workers.
He was there immediately after the flooding, and then-president Obama didn't even go until maybe a week later because he was on vacation somewhere.
Not only did he go and help out, but Trump personally donated thousands ($29,000 ?) to the rescue effort. The people in Louisiana loved him !
I am sure that he will definitely help restore hope and encouragement when he is in Texas, too.
Already, the rescue efforts have surpassed what has happened when we had bad hurricanes before.
Edit to add:
While I was writing this, Ina messaged me. she is doing okay, the water is still going down slowly, and she is worn out. Yesterday, when she went outside to help the people being rescued get into the rescue boat, she was attacked by ants (probably frantic ones), and she said she has a fever today which she thinks if from the bites on her legs, and she is going to stay in bed and rest.