For example you use order-related billing on delivery quantity and create a proforma with reference to a delivery in the background to print a valuated delivery note. At that point you realize that a condition was incorrectly applied. You have not issued a fiscal invoice and you still have all copies of the paper documents (you have not left the plant to deliver products).
You will adjust the pricing in the sales order (you have to do that anyway), but you still need to correct the proforma, because it will be with with the old prices and you need the correct amounts in the valuated note. So you go to VF02 and reprice the proforma (e.g. carry out new pricing). Then you can issue a new printout with the right data. Another case that can happen is when you realize at the point of printing the delivery note that the tax classification of the customer was set incorrectly. You can adjust the documents manually by setting the indicators, but you need the updated info in the outputs.