Including your flight number on your booking means that in the event your flight is delayed, then the rental company will know your new estimated arrival time and hold your reservation for you. Many depots are open 24 hours so this is less of an issue, but some smaller locations close late evening or early hours of the morning.
If your flight is delayed and they have your flight number then the location should remain open until your flight arrives and all bookings have been processed. This still does not guarantee the depot will remain open beyond normal open hours, so if you are very late arriving and you are not able to collect your car as a result, then you would need to make alternate arrangements until your are able to collect your car. In most cases, your airline or travel insurance should assist with associated costs.
If you do not know your flight number at the time of booking, this is not a problem. You can add this at any time by contacting us with your order number and flight number, then we can add it to your voucher. For collections from certain locations, the flight number is not used.