The comments here at the time of my writing don't provide a good explanation of it. There are practical UBI proposals out there that are revenue neutral for various countries that would largely replace income transfers of the existing social safety net, better go look them up. (There is no fundamental reason it needs to be RN but it's a good starting point to understand why it doesn't intrinsically mean more funding).

It's not a panacea, generally it can make current recipients of means tested transfers well off in exchange for eliminating some problems associated with means tested programs.