We know welfare works. It is pretty simple. People need to eat, they need shelter, and a few other basic necessities. When they fall on hard times, not having those things can make it infinitely harder to get back on their feet.
Our current welfare system is a bit of a bureaucratic nightmare and has a stigma attached to it. The nice thing about UBI is it is universal, and simple. The second your income goes below a threshold, you start getting a little bit of money. The second it goes above, you stop. No fuss, no stigma. They are going to need that money to survive anyway, so might as well just give it to them rather than forcing them to suffer the indignities of poverty.
Public policy is a messy business. But I find it odd that this topic is so controversial as it only took me a few minutes of ruminating on it before realizing how good of an idea it is. Maybe it is because I have had a brief encounter with poverty myself, and ever since my anxieties around finances, access to healthcare, etc.. run deep. Or maybe I just realize that a ton of people were born into poverty and due to no fault of their own are now stuck in it. Money might not grow on trees, but it quite literally exists as 1's and 0's in some database. The fact that we could just flip a few bits and instantly make the lives of so many people better, boggles my mind why we wouldn't try that.