For me personally at least, why I really want UBI is for soul-starving needs specifically. My job I fully believe is either net-neutral or hurting the world, and after some amount of looking have not been able to find one that's both clearly positive and not exploited. If I was given UBI I would quit, maybe volunteer to teach one class a day in a high school, or help run a boardgame cafe or makerspace. There would be no worry of putting people out a job since they'd be getting UBI too.
Right now I feel souless. I'm living well but through some glitch in the system that rewards people for writing useless code on a project that I'm expecting will get canned (the last two were thrown away) and spending time on hackernews. I want to be doing the useful things that currently don't pay well enough to be live without anxiety.
I think a lot of people working exploited jobs would also benefit, for the reasons you already know if you want a general safety net. Being able to quit without fear is a lot of leverage and would make things a lot more equal between employer / employee. Current unemployment benefits don't give anything if you quit (instead of fired) and a UBI style no-questions-asked solves this.
Addictions / vices are certainly a problem, and could definitely be worse with UBI perpetually enabling. I don't really have a great answer to this. The current system does not seem very kind to addicts without external support either, so I'm not sure how much worse this would be either way but definitely something to think about. Hadn't really considered that angle before.
Either way, I'm not 100% convinced UBI is the best way to add a safety net. I do think it would have some pretty profound effects on what work people would be wiling to do, most of which would be positive. It's a pretty simple / meme-able demand the same way 'fight for 15' or 'healthcare for all' is, that's really all the pitch i have for it tbh
What sort of changes are you hoping for?
(edit: is there a comment reply limit? if this is the end im happy to give an email or phone # to continue somewhere else. legit this is the only pleasant hackernews comment thread i've been a part of on this or my alt so thank you very much <3)