The U.S. Constitution's 21st Amendment repealed the 18th Amendment, which prohibited the transportation, manufacture and sale of alcoholic beverages. It was passed in 1933, thus ending 13 years of Prohibition. The repeal generated a number of positive effects, such as giving adults the personal freedom to drink again and weakening organized crime's grip on power.
Prohibition did not actually make people stop drinking. Rather, it just made them drink less, and do so surreptitiously. So, when Prohibition was repealed, it made many criminals into law-abiding citizens. This in turn freed up police resources to focus on other crimes. Making alcohol legal made the overall crime rate (including assaults, burglaries and other crimes) go down as well.
When the 18th Amendment made alcohol illegal, people still wanted to drink. Gangs took advantage of the fact that they could supply a high-risk product at a similarly high markup, and were able to become substantially more powerful than they were when alcohol was legal. When the 21st Amendment made people able to buy alcohol at their corner grocery again, the gangs themselves lost a customer base, which reduced their income and made them less powerful. This ripple effect made the streets less violent for everyday citizens.
During Prohibition, people were drinking but they were not paying taxes on it because they bought their alcohol illegally. This was a major reason the 21st Amendment was passed -- in 1933, the country was in the middle of the Great Depression, and the government needed money from taxes on alcohol. So, a positive effect of the 21st Amendment was that it stimulated the economy and provided the government with much-needed tax revenue.
The 21st Amendment gave people the ability to make decisions for themselves again. While alcohol may not be healthy, it is not really up to a third party to decide whether you drink it or not. Each individual adult citizen's health is his business, and the 18th Amendment was promoting the opposite ideal. So, when the 21st Amendment was passed, the American government took another step toward leaving adult decisions in their own hands, rather than in the hands of strangers.