Futures are "written" by the person with the commodity to sell, and sold to either a financial speculator or else to someone who wants the product--in this case, an oil refinery. Sellers/owners do this because they want to be assured of a fixed price for the thing they're selling. The official reason for buying a future is to get a fixed price for something. This allows businesses to plan ahead.
However, since futures contracts are traded on secondary markets, it's possible to make (or lose) a lot of money trading them.
SOMEBODY ELSE: Doesn't it ever backfire?
SOMEBODY: Yes, the market price could fall through the floor and you'd be stuck paying THAT instead of the new, lower price. But at least you know what your cash flow will be.