[brc]

You can't use terms like 'useful' when measuring jobs or output. It's in the eye of the observer.

Apparently nail painting businesses are a viable industry, in my eyes they are totally useless.

Most of the businesses we recognise today would have been hilarious to our ancestors. Paying good money for a personal trainer? Paying someone to wash your car?

This is an age-old problem which keeps solving itself.

[ethbro]

The "eye of the observer" in aggregate is what forms the invisible hand in a free market.

But jobs existing are a consequence of two things: (1) demand & (2) ability to price work output above a minimum.

UBI answers a world where there aren't enough of (2) to satisfy labor supply (whether through automation or increases in efficiency). Because bar repealing minimum wage statutes we're going to reach a point where for {minimum wage salary} there only exist X potential jobs (additional jobs not economically-feasibly offering a sufficient income), and labor pool > X.