It is basically changing the topic when you have no effective rebuttal to an argument/issue.
It also creates a false equivalence between the real issue and the "whatabout" issue.
In this case, GP has no answer to China going FAR beyond any reasonable measure by requiring any tourist to install spyware on their phone so they can access all private conversations, so GP wants to ignore that and talk about public info the US requires visitors disclose at the border.
The false equivalence is created by treating as parallel and roughly equivalent govt actions the requirement to install spyware vs divulging of SocMed accts.
I'm NOT saying that divulging SocMed accts could be a definite threat to a variety of classes of people, especially journalists writing undercover. But even for that specific example, which is worse, enumerating your public SocMed accounts, or installing spyware on your phone, which will divulge far more? Not even in the same ballpark.
In sum, Whataboutism is not only damaging the conversation, it can often also be a method of disingenuous argument.
"There is an increasing trend around the world to treat borders as law-free zones where authorities have the right to carry out whatever outrageous form of surveillance they want," Omanovic said. "But they’re not: the whole point of basic rights is that you’re entitled to them wherever you are. Western liberal democracies intent on implementing increasingly similar surveillance regimes at the border should look to what China is doing here and consider if this is really the model of security they want to be pursuing."
In reality, comparables sometimes are and sometimes aren't relevant. You have to make that argument case by case for it to have any meaning. Invoking a generic word as if it magically decides the matter is just the sort of thing the HN guidelines ask commenters not to do.
To pick an example from another, hopefully distant enough, flamewar topic: if someone complains that dynamic programming languages have runtime errors and someone replies, "what about null pointer exceptions in $static-lang?", it's reasonable to argue about whether and how that is comparable. What's not reasonable is to exclaim "Whataboutism! The topic is errors in dynamic languages. Stop trying to change the subject." That amounts to "you can't say that because I spoke first", and that's not how conversation works. The question of what's relevant is an intimate part of the discussion itself. It's not something that whoever-spoke-first gets to control. Indeed, if anyone did control that, they would have the power to control the entire conversation. Past explanations for anybody who wants more:
https://news.ycombinator.com/item?id=19862258
https://news.ycombinator.com/item?id=19471386
Like GP said, it derails discussions into talking about a different bad actor than the actual one committing the bad behavior, while (intentional or not) minimizing said behavior. In short, just because the bad behavior exists elsewhere does not justify doing the bad behavior, or makes it any less bad, and the act of pointing it out can be a poor attempt at distraction from culpability.