1) Efficiency. A flying car can theoretically take a straight line to its destination instead of having to follow a complex road layout. It might be less efficient to fly per minute, but if you're flying less minutes, it could be a net gain.
2) Reduce congestion. How much fuel in a car is burnt in traffic? With flying cars, congestion will largely be a thing of the past, you have effectively unlimited road just by changing altitude.
3) Reduce urban sprawl. You can get rid of a lot of the roads that plaster the countryside. There will still need to be some obviously. But less.
4) Reduce national infrastructure costs. As above, you get rid of the roads, you don't need to maintain them anymore.
5) Convenience. You don't need a driveway or garage on your house when you can land on the roof ;)
All in all there are quite a few decent reasons for it. Just depends on whether you can beat that efficiency level.
It would be interesting to see if anyone has done calculations factoring in the cost savings from reducing sprawl and the miles of road that are maintained each year.