Answer :

The 21st Amendment repealed the 18th Amendment, which prohibited the sale and consumption of alcohol. It is now generally accepted that Prohibition was a terrible idea because people continued to drink, although now it was with much more dangerous bootleg liquor. Violence was also high. Therefore, the 21st amendment was necessary.
The twenty-first amendment was necessary because it repealed the 18th amendment, which said that people weren't supposed to make or trade alcohol. The 18th amendment made the people angry and rebellious, which is why it was taken away.