So this is the main mode of transportation of the boys, Dean and Sam. They use it to travel across country and also to store their weapons in the trunk in a hidden compartment. It's Dean's pride and joy, a gift from his dad. It's become such a part of the show that's it's pretty much a character itself. Kind of like Serenity on Firefly, the show wouldn't be Supernatural without the impala.
It's just a beautiful car. One of these days I will own one someday.
Oh and the guy sitting on the car is pretty beautiful too :)