Quotes need to be editable so even if all goes well and no changes are requested (customer accepts the offer as is, no change in number, price, discount, invoicing address, contact person, etc), they still may send the purchase order and the PO number needs to be added to the quote. So Finance could issue the invoice using only the final quote, containing all relevant information.
If a different quote needs to be issued for this (current practice), the quote number will change again and won’t have the right cross reference with the customer - they would have the previous quote’s number for their purchase order.