I was filling up for FREE with a full tank of gas voucher that my dealer gave me when I drove out of the dealership. Saves them the trouble of going to get the gas, allows me to select the brand & octane, prevents some jockey from driving my new car to the gas station before I take posession, limits the risk to the dealer of something happening to the car before I take ownership, etc.

That's the same they did it when I bought a Jeep from them previously.