The world saw the effect of radiation that killed people, bodies were vaporized instantly and birth defects caused cancer and survivors were horribly disfigured for many more years to come. Since radiation spreaded throughout the area, food, water, and the environment was poisonous for many years. As
you can see, people are still suffering today. Because the world saw this catastrophic evidence of the Hiroshima bombing, the United Nations all agreed to never develop or use Nuclear weapons again.
After the bombing, Japan was totally wiped out. As a result, Japan surrendered to U.S and later became an ally of United States of America. Japan involved U.S in World War II by attacking U.S. naval ships in Pearl Harbor and therefore U.S. did not have a choice at the time, so they retaliated and attacked Japan with an atomic bomb.
The memory of this event shocked the world and made nation leader to think twice before they attack a country. Having this fear helped prevent wars for a long period of time. Although this may have been unfortunate, the world would not have learnt any lesson from all of this, by not having an example to learn from.