The reasoning is very simple: If you are living hand-to-mouth with unreliable income - so unreliable that you can essentially expect "the end of the world" to come at any time - then you cannot make any future plans around earning and saving; consuming as much as possible immediately holds more value.
But if you have a government-guaranteed income, you can build plans around that - you can pursue the risky project, you can start the startup, you can go to school, etc. One anecdote I read(this was years ago) involved a woman who built up a flock of chickens from basic income. This lifted her out of poverty, since the chickens can pay for themselves with eggs in just a few months.