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Send up a one ton something with control and propulsion so that if it goes to plan it can be directed to the sun or something, not hanging around for eternity*.

It was a test, they'd normally send up a one ton slab of concrete, I think this is a bit more fun, don't you? And why wouldn't he use the opportunity to market his other business?

Pretty much this. Although water has been used as a dummy load in the past and obviously that would be lower impact. As everyone else just said the car hasn't gone into Earth orbit (where the real problem with space junk is) it's gone on a solar orbit roughly in the direction of Mars. This from Space.com:

"He is shipping it out of Earth orbit, so I do not think that there is any risk here," said orbital-debris expert Darren McKnight, technical director for Integrity Applications in Chantilly, Virginia. "The enthusiasm and interest that he generates more than offsets the infinitesimally small 'littering' of the cosmos."

However, McKnight said "it is [a] huge waste of a beautiful car, so I would be happy to take the brand-new red Tesla off his hands, and he can send my five-year old silver Prius into space."

Musk has said it will be up their for a billion years but this is highly unlikely, if micrometeorites don't get it and it doesn't end up being sucked too close to Mars and crash into it radiation is likely to destroy it:
https://www.livescience.com/61680-will-spacex-roadster-survive-in-space.html

From a business point of view I can absolutely see why Musk did this. Sure it's great PR for Tesla but you have to remember that in 2008 both Tesla and SpaceX nearly ran out of money. What saved both businesses (because Musk couldn't choose which one he wanted to save) was the NASA resupply contract SpaceX won on Christmas Eve 2008. After Tesla investors had effectively heavily subsidised SpaceX.

There's a good Bloomberg report on this and the history of SpaceX. Long mind...
https://www.bloomberg.com/graphics/2015-elon-musk-spacex/