Imagine your customers want to reserve some of your limited capacity, before they know precisely what they want to use it for. So you allow them to "check out" empty placeholder orders, to reserve their place in the queue. They edit the order later to add the details - and they very rarely forget, these being some of your most eager customers. (Yes it's an ugly hack, but business types love getting features on the cheap)
Do that enough times and eventually someone will forget to edit the order, and at the front of the queue you'll find an order for nothing. Which downstream systems will quite reasonably not accept, as it's obviously corrupt.