It is usually invoked in the fashion:

- We have this and that reason to do {thing}.

- These reasons are laughable and you still don't have excuse to do {thing}.

- What about when you did {thing} with ever worse reasons stated?

In this case I would say whataboutism is justified, or at least, no constructive dialogue is possible.

It is not justified because however worse the reasons of the other party are, it will never make your own argument true. Your example exhibits the tu-quoque fallacy.

You are right with the last sentence. Unfortunately few people are interested to find out what's true in our world nowadays — for them it is more important to feel justified with their position regardless if its true or false.

The key here is that when talking to an American, "you" means every American since the founding of the US. Collectively, that's an endless mine of material.

When talking to anyone else, "you" means one individual, like Putin, plus one can be sure an American won't remember a single thing he did more than a year ago and that can be dismissed as a digression in any case.