One of my comments from a few weeks ago might answer this:

Alternate histories can make thrilling, engaging, and intellectual stories. A well-researched tale in which some tweaked events extend Roman rule in Europe 200 years could be a wonderful novel. But that is all it will ever be: stories, tales, and novels. The idea of an "alternate history" in any setting outside of creative writing suggests that we possess some ability to predict how human history will proceed. That is an outrageous notion. This is not chemistry, where we can take five known chemicals and mix them together in an unstudied mixture and predict what will happen with reasonable accuracy. Assuming a controlled environment, this really only involves 5 variables with which chemists are rather familiar. If history is a science experiment, it is one with billions of unpredictable human "variables" in an even more unpredictable environment. There is no real value to saying what could have happened because it is so impossible to predict.

TL;DR Alternate histories are fun, and might inspire genuine interest in history, but don't ever mistake anything in them as educational or informative.

Alternative histories are incredibly useful as an exercise to fight against teleological narratives.

When we write about events in the past, there's always a risk of reading later events backwards into the past, using our hindsight to decide what things were most significant instead of listening to our sources. This can be good: we know where things end up, and we usually want to know how they got that way, so we chase the evidence backwards to see how, for example, the Roman empire transitioned into medieval Europe. But it can also become a problem, because an exclusive focus on the origins of later events / realities can cause us to focus so much on how the past becomes the present that we lose sight of what the past was on its own terms to the people living and experiencing it. For example, the fall of the Roman Empire is such a watershed event in our imagination that we assume the barbarians armies in the late Roman empire were trying to destroy it. That is, we assume they were trying to achieve the thing that ended up happening, and we do that because we know the end of the story; it's hard to remember that people in (to stick with our example) the Roman Empire didn't know the empire was about to fall, and were actually not trying to make it fall even as their actions inadvertently led to that final conclusion (this is Guy Halsall's argument in Barbarian Migrations, an excellent book that tries to imagine the alternative futures different people in the late Roman empire were trying to achieve, even while showing the reasons these alternatives didn't materialize).

This danger (assuming that the things most important in a later period were also important in the events leading to them) is what we call teleology. All historians try to avoid it (with varying levels of success), but it's hard to prevent our knowledge of later events from influencing our interpretation of the earlier ones. Alternative histories are really useful as a tool for fighting against this tendency, because they remind us that the things that did, in the end, happen, were one of many possible outcomes (and often were not, in fact, the outcomes that people in the past expected, or were trying to make happen). It's valuable to think about these possibilities because it helps us understand what the earlier periods we study looked like on their own terms, instead of just focusing on the bits of those periods that anticipated or caused later events.

Now, how much of this makes it to our final analysis of events in the past? Usually, not much. But the exercise is important as a way of reimagining the context in which past texts were written, past people lived, and past events occurred in terms that don't derive solely from our privileged knowledge of the future. Most historians (even the ones who dismiss alternative histories) do this kind of imagining to try to escape their own perspectives and figure out how our sources thought the world worked, and where it was heading.