Companies can't spin up new workers in an instant. So for some jobs, they'll need to keep people on staff but not really working. They're there for specific tasks, or when more effort is needed, or for intermittent operations.

A lot of office jobs aren't like factory lines, where output is constant and directly related to input. That doesn't make the job bullshit. It just means that "40 hours per week" is an arbitrary number, the result of centuries of conflict between labor and management, and there's nothing sacred about it.

The amount that the job is worth depends on the job and the company, but it has nothing whatsoever to do with hours served. They need an employee to be prepared to fill that job, and the pay is negotiated on that basis. There's no reason to be expect it to be a straightforward function of prevailing hourly wage and 40 hours per week.