It is the notice sent by the carrier to the receiver regarding the arrival of the item/shipment. It tells the number of packages, weight, address and fee of the particular item.
It is done to ensure smooth delivery of the product so that both the receiver and the seller are on the same line.
It is not an obligation to do so but if done, the consignee needs to know in advance. However, the receiver needs to take the product himself to ensure timely delivery.