Rental car agencies do use the flight info in some situations, especially if a location is not open 24hrs.

A couple months ago, my flight was delayed 2+hrs coming into Milan, where I arrived after the airport Avis rental car office closed at midnight.

They were monitoring my flight and had one employee stay on over an hour after they closed just to hand me my keys and paperwork.

The entire rental car center was completely deserted, aside from the one lone Avis employee. I was very appreciative and glad I added my flight info to the reservation.