I recently spoke with Square Enix's Hajime Tabata at Tokyo Game Show. When asked about the car appearing in several different Final Fantasy XV clips, Tabata noted that the car was very important to the game.
Not only will it be a common mode of travel in the game—he said that the upcoming demo could have up to three hours of content for some gamers, but it'll take "much longer" without using the car—but the players will have it for a long time, perhaps through most or all of the game.
He said that the group is with the car so much that, "You can see the car as another member of the group, because the car is always with them throughout their journey. Think of it as a party member."
Final Fantasy XV's demo is currently expected to be included with Final Fantasy Type-0 HD for Xbox One and PlayStation 4.