If someone wants to work for $1 / hour and someone else wants to hire them for $1 / hour... what right does a third party have to prevent them from doing business with each other?
People need money to live. Some of them need money more desperately than others. Some are willing to work long hours under shitty conditions for almost anything.
So, if you have 1 job, and 2 people need to work to survive, then the salary for that job will approach zero because those people have no other choice.
We, as a society, realize that is a bad outcome for everyone, and so we say "Look, I know you /could/ hire someone to clean your toilet for $1 an hour, but that isn't the true cost of toilet cleaning because we don't live in a truly free market and there are larger factors that might force people to accept that wage against their will. Therefore, we're going to say that $X is the lowest you are allowed to pay to overcome the inherent coercion in jobs."
One of the beauties of UBI is that, at least theoretically, low wage jobs will have salaries that actually come closer to the true market clearing price of the job itself. To be clear, it is likely that the true cost of toilet cleaning is closer to $3 or $4 an hour, but people with no options would take the job for $1...and there are a ton of those people out there. Under UBI there might not be people that desperate any longer and so they would only take the toilet cleaning job if it paid more...closer to the true market value of having clean toilets.
The UBI angle works both for and against worker negotiating leverage. On the one hand, workers know that their entire income isn't on the line if they quit, so they have more leverage. On the other hand, they know that they're getting X from the government, and while they need Y to live, they'll accept work at Y-X.
If people had the right not to work, you'd have a more liquid labor market. If people's basic necessities were met before needing to work, you'd have a more liquid labor market.
Until that day, minimum wage are basic hacks to try to present some of those labor externalities as a basic deserved negotiating position.