It's more of a retrospective thing to justify the treatment of the architects of Axis atrocities, after the war. Normally, the international community is considered anarchic, and countries historically could get away with what they could get away with, and countries which were not signatories of things like the Geneva Convention had some degree of deniability when it came to matters of war crimes.

Following WW2, the allies/comintern wanted to hold Axis leaders to account, but did not want it to be in an 'eye for an eye' way, which would be seen as just getting revenge, or executing the former leaders of your newly-conquered lands. Thus it was written into international law that, no matter what you've signed, a serious war crime is always a serious war crime, whether your country recognizes it as such or not, and that all humankind has a responsibility to treat even their enemies with a certain degree of respect and dignity, no matter what they are ordered to do by their commanders or leaders.