The general arguments are that it's worth sacrificing some of the top end to reduce the severity of the bottom end, partly for ethical and partly for economic reasons. The hypothesis is that in successful Western societies, at least, the majority of value is not created by people who are working solely to avoid dying. And therefore there is no great loss if we remove fear of dying from the list of motivators to work.

The hope is that it can be administered in a way such that it will not greatly harm incentives to be successful either, merely dampen the curve by some percentage: so there are still rich and poor people, but the rich people maybe will only be 20x as rich as the poor instead of 200x. It's still much better to be successful than to be on welfare, both in terms of prestige and social standing, and material comfort (you can think of it as an order-preserving nonlinear mapping). And there are some arguments that it may help economically in some cases: by raising the floor of what bad outcomes look like, it becomes more plausible for individuals who don't have a family safety net to take risks.

As for what possible societal benefit, the Scandinavian societies are fairly nice places to live, partly as a result, though making like-for-like comparisons in economics is notoriously difficult. They certainly have lower levels of severe poverty, crime, homelessness, etc. But there are many books arguing for and against these views, so I'm not sure I can convince you in a comment.