> I'm all for helping people reach the bottom two rungs through targeted spending. UBI is indiscriminate, if our goal is the bottom two rungs specifically why not target the spending for those instead of just giving free money?
That does sound like conventional common sense, but this is a situation where common sense actually isn't. It turns out that attempting to target too specifically can quickly turn into a logistic nightmare.
(Next part YMMV, this is very high level, see your local laws/systems for details)
Many welfare systems already just give people money directly, because the assumption in western countries is pretty much that most people over 18 are quite capable of managing their own money as is, and can do so better than any centralized socialist committee.
Those who cannot might be placed under adult guardianship/ward of the court (depending on where you this system has many different names), at which point in time the guardian/ward can (assist in) mak(ing) fine grained decisions.
Some systems do have some amount of limited targeting, and most of them are means tested. Most modern welfare is a kind of Conditional basic income.
One path to understanding why people propose an UNconditional basic income is the discovery of the welfare trap: The means testing that your common sense tells you is supposed to ensure that only people who need it temporarily get welfare, actually traps them in welfare semi-permanently.
If you're not working, your expenditure is lower as well, since you don't need a car or tools, and things don't wear out as much. This can easily be a swing of something like €500-€1000 or more. Say you are a rational actor, you have €1000/mo welfare [1], and you can get a job for €1500/mo [2], should you take it?
Answer: No. You will have less net remaining at the end of the month, or might even run out of cash entirely. And... unfortunately many welfare systems take savings into account wrt means testing as well, so you don't have a runway. Thus, the trap is complete. [3]
If you drop the conditionality of the payments, the trap evaporates. So the outcome of giving people money unconditionally is ironically that more rational actors will actually get a job and not need it anymore. [4] [5]
Ok, that's answer one, and it's getting pretty long. I'll put other answers in another comment, if I get to it.
[1] Typical amount for a single adult in .nl
[2] This is presumably one reason why EU nation minimum wages are often a bit over Eur 1500. This does help mitigate somewhat, but it's still a dicey proposition.
[3] This is obviously something of a toy problem. IRL means testing and subsidies can be a bit more intelligent, but when researching you'll find that the underlying issue definitely there.
[4] This is somewhat borne out in real world tests. However, the tests I give most credibility to -while some people do start working- also see some people dropping out of the commercial working population to put more effort into childcare and/or go back to school, as well as free riders (because there are always free riders). The net result is that roughly the same amount of people keep working. One can argue over exact definition of working population, quality of working population, effect on crime rate... etc. But if you'd expect the amount of workers to drop precipitously, that doesn't tend to happen.
[5] This -then- also explains how you pay for UBI. You can tax people post-hoc once they're working again. This is how a similar proposal [6] by the Nixon Administration would have actually ended up working, sadly that proposal didn't pass.
[6] https://en.wikipedia.org/wiki/Negative_income_tax