>  “you are saying I am doing X bad thing? What about Y bad thing you’re doing, let’s discuss that instead”.

often times X is actually bad but human nature is such that no one actually cares about it while wanting to appear to when locked down on it. for this reason arguing about X directly is bad optics.

whataboustism is basically an effort to draw attention to the selective enforcement of norms, laws, morality, whatever. hypocrisy is implied.

It can also be used to derail a conversation. I.e. "I find subject X unpleasant, so I am going to imply that you don't care enough about subject Y, which has a passing similarity to it, so that the subject of the conversation turns to Y rather than X. I do not actually care about subject Y, but I won't say that out loud". See also "concern trolling".