There are a lot of costs associated with living without a stable revenue: emotional, social, etc. It's not only the bare financial value that one receives. If you're jobless, or without a stable / guaranteed revenue, your life changes on so many levels (obviously, for the worst).

I think the mere fact that survival is somehow guaranteed will lessen some of these burdens. And it makes sense from an economical point of view: if productivity will increase as a result of using AI and robots, then there will be some extra profits. Distributing a part of them equally to all people living is the logical way.

Think at UBI as it is oxygen: no one can actually live without it and yet, there's plenty of it so no one will die immediately after being born.