Safety. When people see no other way out they can do anything. Making sure everyone's basic needs are cared for makes it a safer and more stable society for everyone.
You're being downvoted but it's a good question, and there is a great answer.
The more people can fully help themselves, the less I have to help them, the more of my production I get to keep.
The increased standards of living of all people, increases the potential for all. More wealth in the economy, more capital that can be invested, more income, more potential jobs.
Speaking simplistically, if my neighbor is doing better, now we can trade. Whereas perhaps he previously, in poverty, could not afford to trade with me. I just gained a new customer and perhaps he can fulfill a demand I have the other direction.
The human mind in general is extraordinary when taken to its potential. Even people with median IQs are capable of tremendous productivity and contribution when they're unleashed.
Violent crime in the US has declined dramatically at exactly the same time the US has dramatically expanded its welfare state. It's highly unlikely that's a coincidence (or solely to be chalked up to removing lead from the environment). People that are less desperate are going to be less violent, they will commit fewer crimes, it will produce a safer society.
At the far end of the scale, I dramatically benefit from there being more Einsteins, not less. I benefit from more Edisons and Teslas, not less. A stronger society ensures that less Teslas die in childhood, and reach their full potential rather than otherwise languishing.
How many great inventors were lost in Mao's genocides and famines? Or otherwise languished in 40 years of extreme, forced poverty with minimal education and sustenance? Plausibly a lot.
The higher the income of the bottom ~2/3, the less welfare they require, the lower taxes can be, and the more they can net contribute to society. The lower taxes are, the more capital becomes available in the private sector to save and invest. As an example, if you considerably move up the incomes of the people in the 30% directly below the median (not the bottom 20%), then they require less of the tax revenue to go to them for social welfare purposes, and they can become greater net contributors to helping the 20% at the very bottom (whether through paying taxes or charity).
At national levels, this has enabled for example the dramatic reduction in aid that needs to go to a country like China or South Korea, as they've climbed out of poverty and into greater prosperity. That aid can now go to the other remaining poor nations, and those countries like China & South Korea can become considerable net contributors to global aid. The whole process dramatically accelerates in a virtuous cycle. The exact same concept applies within a country as well, when it comes to lifting people out of poverty such that less aid needs to go to them and that those people can become net contributors.
I don't agree, you can ask for redistribution so the weakest in society are helped.