You never know if there will be a bad year. In the bad years the price of the stored will be much higher than if it had been sold during the good years. It is unnecessary if there is a Joseph who knows how supply and demand will develop. But Joseph is unique. There is a decisive difference between a slavedriver who can force and an employer who can offer employment.
Consumers buy products produced by companies who sell the best product at the lowest price.
This bears a certain relationship with how the production is organized and how expensive it is. Some of the consumers may be very poor. To believe that all the poor are poor without cause is probably as wrong as believing that all the rich deserve their wealth. To succeed in business you need both luck and skill, or at least not bad luck. The pastoral letter eventually moves on to man's ability to create.