The first launch of the new Falcon Heavy will be sending up a Tesla Roadster owned by Elon Musk as its payload instead of something more useful.
Why spend all that money and effort sending up a car? Why not send up a new segment to the space station or some equipment that is not critical but useful?
The very first start of a new rocket is a risky endeavour. Since the system is put to test for the very first time as a whole, all kind of things can go wrong and chances are that the rocket doesn't make it into orbit. So a cheap, unimportant payload is needed for the first launch. You don't want to see something worth billions of dollars and having cost years of development to blow up. Instead of launching a boring piece of concrete or some other weight, SpaceX decided to make a fun publicity stunt by lifting Elon's Tesla Roadster instead.
It's not the first "silly" payload SpaceX has sent into space, either: their first Dragon test flight transported a barrel of cheese, as a reference to a Monty Python sketch.
Why not deliver something useful to the space station like a new living segment?
Many, many reasons.
Even if those space agencies had asked SpaceX to use this flight to send something to the ISS, and even if SpaceX was willing to sacrifice one of their Dragon spacecraft, doing so wouldn't make sense. It would not showcase the prime purpose of the Falcon Heavy, which is to push payloads well beyond low Earth orbit.
When SpaceX launched the Falcon 1, it took them 4 times to successfully orbit the Earth. The most similar rocket to Falcon Heavy, in terms of the number of engines, the N1, made 4 attempts to reach orbit, all of which failed. The more engines, the more complex things are. The chance of a failure is extremely high on this launch. No one would be willing to put a valuable satellite in to the maiden flight of this vehicle.
The Falcon 9 initial test was a Dragon qualification test. This actually made some sense, as the Falcon 9 was required to lift the capsule, and testing both at once was minimally risky. But there is no similar payload at this time. The risk for this particular configuration is very high, only the Delta Heavy has a similar configuration of any current rocket system.
Bottom line, a payload was needed that wasn't that expensive, but could show off the capabilities of the rocket. This test fit the bill perfectly. By trying to launch to escape Earth, it can show the full power, and if it fails somewhat, then it will still be successful. And the mass is considerable as well. A useful payload would cost more then the rocket most likely, and just isn't really worth it to SpaceX.
Given the points made in the other answers (that any payload on the first launch is at high risk of loss), it seems to me there's still something useful you could do lift on a test-launch like this:
Low value but high mass raw materials for something that will be assembled in orbit. Or water. Water is heavy but useful (as radiation shielding, and for humans to consume), and very cheap. By the time you build a robust container for it that won't slosh around and unbalance the Falcon Heavy, it's still pretty darn cheap.
This means the Falcon Heavy doesn't have to go near any existing valuable thing (like the ISS or any satellite) with the Falcon Heavy itself, or risk a Dragon orbiter for the rendezvous. Future valuable stuff can be launched (on better-tested rockets) to match the existing orbit where the raw materials are.
Fuel (e.g. for a Mars mission) would be the obvious choice, except that @StianYttervik points out that an explosive payload is a Bad Thing, making failure on the launchpad even more dangerous.
Presumably SpaceX thought of these possibilities, and still decided to do a publicity-stunt launch instead. They're a commercial company, and the value (to them in an economic sense) of this publicity stunt is well above zero.
Presumably it exceeds whatever they thought they could get from launching low-value raw materials. Or maybe nobody was interested in paying to have raw materials launched as the start of an orbital-construction project any time soon!
As others have commented, proving they can get a payload onto a Mars trajectory is pretty cool.
There was something useful... A commercial in the superbowl cost 5 million for 30 seconds! How much advertisement has Elon Musk just gotten for the Tesla Roadster brand?
$250,000 for days and hours of video, news article, and picture time is an insanely good return on investment as far as advertising is concerned!
Plus the Tesla Roadster is now the fastest car ever!
The payload is immensely useful.
As noted by others, a traditionally "useful" payload costs a very large amount to provide, the risk is high, and the ISS is not an available option at this stage.
Instead, SpaceX, Tesla and Elon are achieving a win-win-win solution.
A, or indeed arguably THE, target of Elon's efforts is to establish a Mars colony. If successful the car will be placed into a Mars (near) intercept orbit. It's lifetime is likely to be "indefinite". The car will (they hope) serve as an ongoing billboard advertising SpaceX's aims, Elon's involvement and the Tesla product. 100 or even 1000 years from now, and long after ISS's brief lifetime, barring various unhoped for eventualities, the car will still be there advertising the prowess of SpaceX, Tesla & Elon. Far better than a lump of concrete or a cloud of ice crystals.
Mars related:
While a Mars injection orbit was planned the launch has produced an orbit with apogee beyond Mars orbit - expected to be somewhere near the asteroid belt.
Guardian: SpaceX oddity: how Elon Musk sent a car towards Mars
Syfy: NEXT STOP, THE ORBIT OF MARS: THE FALCON HEAVY IS SET TO FLY TODAY
Musk's vehicle won’t be going to Mars, or even orbit around Mars. In fact, it could be several million miles away from the Red Planet. Rather, this launch would put the car into the kind of orbital loop that brings it close to Mars and Earth over and over again.
The Falcon Heavy launch would place the Roadster into a heliocentric orbit, meaning that like the planets and comets and so on, it will be orbiting the sun. More specifically, the Muskmobile will go into a type of heliocentric orbit called Trans-Mars injection, which it is the easiest and least energy-intensive way to move objects back and forth between Earth and Mars.
At specific moments every two years, the conditions are right to fire up the spacecraft’s engines and slide from one orbit to the other. A future transportation network that supplies a Martian colony would benefit from this kind of scheduling. Think of Musk's car like a city bus on a scheduled run through the solar system, slung this way and that to take advantage of the gravitational pull of the sun to make it easier to get into Earth or Martian orbits.
They're testing if they can use Falcon Heavy to send stuff into a heliocentric orbit, not an earth orbit. So anything going up will be out of our reach for a long time.
The only thing that can be useful in those circumstances would have been some autonomous space craft. And those are too expensive given the odds of mission failure. So the next best thing is a big publicity stunt that will get space-flight back into peoples attention.
This question is a specific situation that has been answered by the general question of What is simulated payload and why they are used
Short Answer: Satellites cost millions of dollars, especially on a launch with the risk involved with trying for a solar elliptical close to Mars, if not Mars orbit itself. A payload of water however, is cheap. And Elon Musk also owning Tesla, why not send something that in many ways will cement his legacy forever. Even a $75,000 car is significantly cheaper than a purposeful payload.
Also considering the actual happening of what happens when a launch doesn't go the way you want, Look at what happened just a month ago. SpaceX botched launching a classified satellite, code named Zuma, into orbit of our own planet. Who knows just how much that satellite cost, let alone the bad image it gives. Now, SpaceX is claiming their side was a success, and it was Northrop Grumman that botched the job.
