Exactly that: it shows a level of work, discipline, and intelligence. It doesn't cause any increase in the ability to work, be disciplined, or be more intelligent. Society is better off if this is something like a prestigious high school degree, rather than a Bachelor's.
I can't hire for things that 'cause' such increases, I can only use presented-information to make a best-guess about how an employee will perform.
Having a degree is a good indicator that you'll work hard and be disciplined. If I have to break a tie between two candidates, things that show levels of work/discipline/* will help me break the tie.
Again, that's perfectly fine and reasonable in individual situations. It's also not an argument for society getting more education as a whole. The tie-breaking for you is the same whether it's between high school diploma vs bachelor's degree, or a bachelor's vs master's. Making the high school degree holder get a bachelor's and the bachelor's get a master's doesn't help society as a whole.
I agree that a rigorous high-school education could just a well serve as a filter as the higher ed studies do today. Unfortunately, high school is mostly low-quality, low-effort waste of time these days, so finishing it does not signal much to the employees (maybe with exception such as Eton and other elite private schools).
