If a customer says that they haven't received their tickets then you may want to generate a new Barcode and resend them the ticket. Doing this ensures that the existing barcode that was originally sent out can't be used, preventing any potential fraudulent use.
There may also be instances where Print Rules have not been set up yet for the Delivery Method, and/or some orders where you do not want to add a Delivery Method. In which case you will need to generate the Barcode on the Order page and send out tickets after the order has been completed.