The point of the ICC trial is to investigate, in the name of the signatory states, if the crime of genocide is being committed by another state, signatory or not. Then, it is up to those states that recognize the court to follow through with the impartial decision.
Today, France or the UK or Germany or my own country, Romania, can claim that collaboration with the regime in Israel and/or the one in Gaza is not problematic, because no court of law has found that they engage in genocide. If the ICC finds that there is a credible enough case that they would issue arrest warrants for the highest ranking politicians of those two states, then this would be no longer acceptable. They would have no more fig leaf to hide behind: a court we recognize as legitimate has made it clear that there is a plausible case for genocide taking place, and so we must (if we abide by our own laws, treaties, and logic) end our collaboration with these regimes and seek to bring them to justice.
And I find it very interesting that you're comparing the Palestinians with Nazi Germany, and Israel with the Allies. In fact it was Israel that invaded and is currently occupying the Palestinian territories, just like Nazi Germany did with so much of Europe, and it is Israel conducting a genocidal attack in Gaza, not the other way around. Palestine is much closer in this scenario to occupied France or Poland, striking back occasionally at their occupiers. Unfortunately, they are lacking powerful allies like those two countries had to help them free themselves, and they have also been radicalized and are choosing to murderously and condemnably attack the civilian population of Israel instead of focusing on military and state installations as those revolutionary forces mostly did.