As you said, once you have scheduled one bill run, you do not need to take care of it after that. The scheduled bill run will be executed at the scheduled time. In some cases, merchants can schedule a payment run to be executed later than the bill run to automatically pay the invoices generated by the scheduled bill run. In this way, with the help of scheduled bill/payment runs, the merchant can automatically generate invoices and collect payments from their customers. So the benefit of the scheduled runs is to provide convenience to the merchant to manage their business as well as the efficience.
To your question, the Ad Hoc bill run might be helpful if you want to generate the invoice for one shot instead of repeated and regularly.
Hope that make sense.
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