This is explained in an interesting book on futures and options trading.
The Japanese invented it to level out the price of rice throughout a year.
At harvest time, there was so much rice available to buy, the price dropped through the floor and the farmers couldn't make any money at it.
In the winter, there was very little rice left to buy, and prices were so high, people couldn't afford it.
Enter commodities futures trading.
Speculators would pay farmers to deliver rice at a later time in the year.
This would be more than the going price at harvest, less than in winter.
The speculators would take delivery later and sell the rice at a higher price than what they paid for the future delivery.
Farmers could now survive and so could consumers.
