Like any commodity oil can be used as a hedge against inflation. So much like smart investors have positions in gold & silver, they also take positions in oil.
The best part is they can front run the bull market, and due to their ability to control supply either they're an exporter in the case of Saudi Arabia, producer like Exxon, or investment banker with insider information Goldman Sachs they can also front run any bear market.
It is the ability to front run that creates the market fluctuations in prices that greases the skids of liquidity. But the one constant we all know is oil, like gold and silver is a finite resource, and capitalism's basic premise is ever expanding growth. There will always be a break even point where we can no longer have growth beyond the price of oil. It is at this point and slightly below where we find the price of oil. Inflation is merely the capitalist producers profit realized on this price discovery.