BELNR is not required however it is useful. Let us say a customer place an order and as he is not sure he place the order again. Now there are 2 orders you are not sure if there are two IDocs for same order.
With BELNR, it has order number of customer system and you can add an optional field with Order Number and incase same order is used it can raise a warning message. You can also use session id to check for duplicate posting if you don’t want to use this.