> In the (a) case where they are actually having drivers sit idle, then they are reducing the throughput of their system significantly.

If there are 50 deliveries per driver per shift and I want do deliver everything 5 minutes later, I don't need the driver to idle for 50 × 5 minutes.

The driver only needs to start the first delivery 5 minutes later, at a time cost of 1 × 5 minutes. Then they finish it 5 minutes later, and hence start the second delivery 5 minutes later, without standing idle between deliveries.

And if I pay the workers per delivery, that 1 × 5 minutes of initial delay doesn't cost me anything except worker morale.