I hear some people complaining that this is a waste of resources. These people are ill informed. Below I debunk this.
1. The FH needed a dummy payload for its first launch, so Musk's old Model S that likely just sits in his garage is serving a purpose. A dummy payload was going to happen, so might as well throw something interesting and useful, see point 3.
2. A test launch isn't a waste of resources if it demonstrates future capability, especially if that is capability is deep space or Mars.
3. Its a PR stunt for SpaceX and Tesla, it will get those companies alot of attention as the launch comes and goes and we follow the progress of the car. People tend to relate to cars on a personal level so it's guaranteed to attract attention. So it serves a purpose for Musk and his companies.
4. In a cosmic perspective, its funny and random as hell.