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.
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.