What does pine tar do and where do you get it? I bet it is flammable. Does it put extra heat on the ball when you light it up?
It helps them grip the ball. Batters sometimes like for pitchers to do it because it lessens the chance of them being beaned.
You put out the word on the street that you're in the market for it. You get an anonymous phone call setting up a meeting place. You put on big sunglasses and a cap, take out a large cash withdrawal from your bank and you know the rest...;)
Seriously, though, I think it adds friction to your pitch, so you can do some fancy throws.