Interesting question. I can only think of why a company would want to run an ad-hoc Payment Run. An ad-hoc Bill Run might be necessary if for example, you don't have the Usage data collected in time before your scheduled Bill Run takes off. If that's the case then you can use Zuora's Bill Run API to create a bill run ad-hoc, avoiding the need for human interaction.
E.g. As a business I need to collect all my usage data before running a bill run. Once usage data import is complete, then use my message bus (e.g. Mulesoft, WSO2, Talend and the likes) to trigger a call (SOAP or REST APIs) to Zuora to create the bill run.
Now the Payment Run is another matter, and I've seen a ton of use cases around running payment runs that Zuora haven't yet addressed, but with Bill Runs, there's an API and you can deal via implementing whatever Biz Process you have.
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