There's different types of work, the on duty time you describe is what I would call being paid to be available to work during certain hours. My work is like this, I do 9-5 or similar hours, any jobs come into during that time, I work on them as needed. Some days are quiet, some very busy, work is somewhat elastic while time and hourly rate is fixed.

This applies to a lot of jobs these days, even from something like retail or hospitality worker or at the more extreme end you have the likes of an on call engineer or firefighter, mostly being paid to be ready to respond to the need to work, but not being guaranteed that it will happen. This type of work doesn't really respond to more hours being more productive. It may to a certain extent that if the shop is open long maybe more customers will come in but it's not guaranteed and there may be diminishing returns as you are paying employees for their time so you need to be making or saving more money having them available to work.

Most people still conceptualise work as something more like factory work or mining or labouring even something like legal work to some extent, where you have a set amount of output each hour and so more working hours should mean more production output to a certain extent. In this case work, hourly rate and time is fairly fixed and not that elastic.

Then there the type of job where the work is fixed, but the time and hourly rate can be very elastic, this is more contractor, startup, even parts of the military, where you need to complete a task or objective maybe to a deadline but the the time or cost can vary significantly depending on what's important. This is more the business owner, sole contractor or equity earning employee where they can potentially get a bigger payout by putting in more effort or else working more efficiently.