Car rental companies ask for your flight information so they will know if your flight has been delayed, or if there are any issues. By doing this, the car rental company can hold your booked car for you.
If you decide not to provide your flight details, you can still pick up your car from an airport. The car rental company will not know when you are due to land and will not be able to see if you flight has been delayed or has landed early. If your flight is delayed, the supplier will not know and you will need to call them as soon as possible.
If you give the supplier your flight information, they will know if your flight has been delayed and can hold your car for you. In the event that your flight is delayed, notify the car rental company as soon as possible. Delayed flights occur often, so there is no need to stress. Just let the car rental company know as soon as you do. If you don't get the chance to speak to them, they will be able to see it in their system.
If you find out you are flying into an airport later at night and your flight is delayed, make sure you let the car rental company know as they may need to keep the counter open for you until you arrive.
In the event your flight is canceled or re-routed, you may not be able to collect your car rental and may have to re-book a new car rental. When there is a small delay, the supplier will most likely be able to hold your car for you. If there is a major delay, the car rental company will be able to sort something out with you.
Alternatively, if you arrive early, you may not be able to pick up your car until the specified pick up time, especially in peak travel times. If they do agree to give you your car hire early, make sure to check if you will be charged extra. If you will, it may be better to simply wait for your correct pickup time to avoid any unexpected costs.