One way I will stick up for Alternative History to be usefull is when the goal is to attempt to shed light on actual historical events. Sometimes by posing hypotheticals, even unreasonable ones, it helps us to put things into context.
I'll give two examples.
I was discussing with a professor of mine how much Wolfe Tone was a Nationalist, and how much he was simply a republican. And I posed that we imagine for a moment, a total republican victory, in whatvever form, in Britain, that saw just as dramatic change in all of Britain as was seen in France, and Tone proposed to Ireland. Abolition of Nobility, Freedom of Religion, etc. etc.
Under such circumstances, would Wolfe Tone, and would his popular republican base, have wanted to sever the link with Britain? We paused for a moment, and decided it was extremely unlikely. So even though it is an impossible scenario, it tells as a little about Wolfe Tone.
Another deals with that great motherload of historiographic debates, Nazi Germany. We were discussing how central Hitler was personally to the NSDAP's success. And after a bit of muddling about and a bunch of 'I dunno lols' from all sides, I posed the question: Can you really Picture anyone else as Chancellor? Goering was only a vague possibility, and seems like he couldn't get into power on his own. Goebells was too unlikable. Himmler was right out, not even possible. Rohm could never even be presented in party politics. The Strassers are too bookish. That leaves...who exactly? Such a dismall showing by the rest seems to point to Hitler himself being very important for NSDAP success.