You must have had a subscription to What if...? comics when you were a kid. You sure do love counterfactuals. Maybe British colonialism would have done for North America what it did for Africa? Maybe the United States in the latter part of the 20th century more like South Africa than Canada. Maybe if the American revolution happens in 1915 instead of 1775 we find ourselves allied with German nationalists in the late 1930s. Maybe in the 25th century Isiah Thomas will be viewed as the best Knicks GM in history.
Assignment desk: Explain the value of counterfactuals and your affection for them.
My affection for counterfactuals and my sense of their value derives from when I took Richard Heck's class on "Realism and Anti-Realism" and we did a unit on counterfactuals. For that segment of the class we were assigned David Lewis' On the Plurality of Worlds where I was first exposed to the argument (much less controversial than Lewis' conclusions about the metaphysical status of modal claims) that there's an intimate link between talk about causation and talk about counterfactuals. My thoughts on this matter were further influenced by when I took the final course Robert Nozick ever taught which was on the philosophy of history (some of Nozick's thoughts on the matter are reflected in Invariances).
At any rate, among historians talk of counterfactuals is in a bad air. But philosophers generally find it pretty uncontroversial to say that (modulo certain complications) causal claims can generally be translated into causal claims. So you might say that the Casey v. Planned Parenthood opinion came out the way it did because David Souter turned out to be a moderate, rather than a conservative, replacement for William Brennan. Alternatively, you might say "if Souter had turned out to be as conservative as Ted Kennedy feared, the Casey decisions would have gutted constitutional protection of abortion rights." Now you can't say these things are precisely the same, because maybe Souter would have been a wingnut who got run over by a bus the day after his confirmation etc. etc. etc. but in a commonsense way the "What If?" question is just a vivid way of thinking about causal claims.