Punish for aggressive acts--the trials were an international means of punishing those involved in violent and unjust acts leading to and during World War II.
German trials in Nuremberg, Germany and Japanese trials in Tokyo, Japan.
Holding the trials in the countries of the Axis Powers demonstrated the power of the Allies over the defeated countries, but also put the leadership on display in their home countries to work as a deterrent for supporters of the defeated governments.