What is the fundamental logic difference MAS uses when entering line items to sales order that exceed available quantity?

To be clear, suppose you're entering a sales order line item that has an available quantity of 5. If you enter quantity 6, you are prompted (among other choices) to first/default Accept Quantity Entered option –documentation says this will disregard the available quantity and accept the quantity entered. This option is available only if the appropriate security is set up. Alternatively, the last option is to Backorder item—documentation says this will back order the amount of this item that is not available to fill the order.

None of the documentation I've found indicates the difference--aside from the fact that backordered items will show up on the backorder fill report when a PO is received with the backordered item(s).

I guess my real question is what scenario would make sense to choose the first option--to 'accept quantity'?

I can't think of a reason you'd want to put an item on an order and not ship it out when it is available, let alone make it the default choice. So, it's making me think that perhaps I'm misunderstanding a core philosophy of the open/back order function.