Prohibition was a useful way for The Establishment to control dissidents in the early 1900's. The dissidents at the time were largely immigrants, Italian and Irish especially, plus others from Eastern Europe who differed from the WASP establishment by appearance, religion, language, customs and morality. They were muscling their way into electoral politics, labor unions, criminal organizations and other important sectors of the economy that the WASP Establishment of the time wanted kept under their control. Most of these dissidents did not have the Puritan taboo around alcohol.
What could be more convenient that to pick up a troublemaker for possession of alcohol; if they were arrested for their politics, they might get more support. But alcohol, thanks to Carrie Nation and other Temperance Crusaders, was known as the Devil's Friend; causing wife abuse, child neglect and a whole assortment of similar evils. The Temperance Crusaders chose to attack the symptom rather than the more politically charged causes: long hours, employer maltreatment, low wages, and miserable working conditions.
Today's Prohibition, called the War On Drugs, is slightly more sophisticated but serves the same purpose. About half of all drug arrests are for possession of marijuana. And a huge portion of all drug arrests are minorities and young people. What a wonderful way to keep them quiet! And what a wonderful way to remind them of who's in charge, since they can be stopped almost anywhere, anytime by the police for a drug search!
And the best part is: since police work has no effect whatsoever on reducing the flow of drugs and police departments make a profit by confiscating the perpetrator's property, it is a self-perpetuating cycle.
Of course, if drug abuse were treated as a health problem as in many European countries, all these benefits to The Establishment would disappear overnight.
I do not think we need to worry about this in the near future.