Good idea in concept. Poor execution makes it doubly troublesome.

Whataboutisms

In the current political environment, people will sometimes try to dismiss something that Trump is doing and say Obama (he is the name most commonly used) did the same thing as though that would justify the actions of the current President.

There is value in looking at other,similar situations and looking at the reactions of people to see if they are similar and what was being said or not said at the time. This reflection is useful because we can test ourselves for bias and over reacting.

That said, whataboutism is when the comparison is a false comparison. For the comparison exercise to be useful, the historical example needs to be relevant before any conclusions can be drawn. By relevant, the historical example needs to be similar:

1. In the material facts. For example, saying that Obama locked up kids is similar to what Trump did is factually inaccurate. Obama was often dealing with unaccompanied minors who were much older and who were flooding across the border at one point. They were not with their parents and needed to be placed with relatives here and that took time. Trump on the other hand was taking kids, including infants, away from their parents. (Very different).
2. In the context in which the events occurred. For example, during the Ebola scare, Obama confined people exposed to the virus to their homes (on the honor system) until they had passed the incubation period. If a President later decides to confine people to their homes in a different situation, one that does not involve the exposure and possible spread of a deadly virus, pointing to Obama’s actions to justify the action of another resident would be a “whataboutism” — a false comparison being used to obscure or justify something materially different.

Like I said before, comparisons are useful when they are relevant. They are a good mental check when things are getting emotional.

The use of whataboutisms is damaging in two ways. First, it tries to justify something using illegitimate means. Second, it actually works to reduce the credibility of reflecting back on a truly similar situation — which is a good thing to do.