It is important here because it takes away the moral high ground. It is also important because those who did it previously have not faced any repercussions, so why would anybody hesitate to do this.
"Whataboutism" is easy because it's purely syntactic, every time someone says "What about ...", you are allowed to accuse them of it--instead of having to confront, in good faith, an implied accusation of hypocrisy.
It's like Orwellian newspeak, but the new word is less expressive and more likely to confuse the disagreement.
Indeed, whataboutism itself is easy because it's purely distractional: "but Y is a hypocrite!" a defender of X may shout, when the topic is not Y, or their hypocrisy, but X, and what X did.
If your argument is that it was okay for X to do the thing, then you should be able to affirmatively say outright "I think it was okay for X to do the thing", and convincingly explain why, on a moral basis, it was okay for X to do the thing, without bringing anyone else into it.
Otherwise, it's like a child whining "but Stevie took a candy bar!" – maybe, maybe not, but that doesn't mean it's okay for you to take one, even if you think that makes someone a hypocrite