there are bad equilibriums where two people could do something for each other but they do not because they don't have money or a way of directly interacting with each other to barter. like if one guy can make shoes all day and another guy can make shirts, unless they have a way of trading with each other, they're going to end up not wearing a shirt or shoes.
if, however, you give both of them "free money", they will start buying shit from each other and in turn getting money and more importantly, making shit. the key part of economics that most people don't understand is that the purpose of money is for it to change hands. money changing hands generally represents objects or actions of value being created which is a net positive to the human race. money in a pile represents nothing. this is why usury was considered a sin until. no value is created when you're paying somebody a debt, and any argument about usury providing liquidity or capital via debt is specious because you could also have a system that does the same thing without debt.
so anyway, a UBI starts the gears turning because people will make and do things in exchange for money as long as there are people with money who will buy those things.
but what is the cost of a UBI? well, it's basically a form of inflation that disproportionately targets rich people. it's a wealth tax and a high-income tax. which is fine in our current situation. in other words, UBI devalues existing money which makes people who have larges amounts of money effectively poorer relative to normal people.