The problem is usually timing, not the flight itself
Imagine you are preparing for a holiday in Europe. Your visa appointment is on February 18, your proposed departure is April 12, and your passport renewal appointment is scheduled for early February. You know which cities you want to visit, but you do not yet know when the new passport will be ready.
The temptation is to buy a flight immediately, print the confirmation and hope every detail will still be correct. Another temptation is to wait until the last minute, when your preferred flight times may have changed. Neither extreme is necessary for planning the itinerary.
A better approach is to separate the stable facts from the uncertain ones. Your legal name, destination and approximate trip dates may already be known. The new passport number and expiry date are not. A booking request should not be filled with guessed document numbers just to make a form look complete.
What actually belongs in a flight reservation?
Airline bookings commonly identify passengers by name and itinerary. Passport information is often collected separately as Advance Passenger Information. British Airways explains that travelers can add such information through Manage My Booking, while KLM offers a way to scan travel documents before departure. That tells us something useful: a passport number can be part of the travel-document workflow without necessarily being the defining feature of the original reservation.