The need for proof of delivery varies from trade partner to trade partner. Most trade partners require proof of delivery principally as an audit requirement to prove order authenticity. Hence a receipt from a customer becomes integral as it acts as proof that the order dispatched was delivered and so the revenue booked on the delivery is legitimate. In a non-eXchange environment most of the times proof of delivery is assumed, which leads to speculation in terms of revenue booked.
Considering that all trade partners are on the eXchange, from principal to the end customer, every time an order is delivered to the intended party, the transit and post transit receipt is automatically generated, acting as proof of delivery, authenticating the revenue booked.