Another benefit to UBI is the security it would offer even to people who don't need it at the moment. I know how precarious/nonexistent the "safety net" is in my country so I am compelled to save and invest all my excess income, building up private stores of wealth against the risk of future catastrophe. If I knew for a certainty that if I were to lose my job or my ability to work I wouldn't be left entirely bereft of outside support then I wouldn't need to save so much--I would then instead spend it, and put it back into the economy. I guess investing is putting it back into the economy, but I think buying more shares of an S&P mutual fund is less beneficial to the whole than if I simply dined out three times a week and tipped well.
But as long as I'm in the position where if I get too sick to work I lose my income and my health insurance, have to burn through my savings, then die homeless you can bet I'm going to be hoarding my wealth and not spending it where others benefit. Another hundred shares of something Vanguard, please, and the waitresses can weep themselves to sleep in their cars.