Here's our use case:
We sell multi-day Juice cleanses, which have a complicated delivery process. We deliver fresh pressed juices every two days of a cleanse, such that in order to process this for fulfillment, we split our orders---one for each 2-day delivery---and modify the Line Item Titles indicating which days of the cleanse are to be delivered.
We started this process in a manual store and then were surprised the same ability to modify an order's line items is not available in orders from Online Marketplaces.