There are at least two traditions of Alternate History in speculative fiction. One, a very old genre sometimes known as “What If”, is perhaps the closest to “respectable” scifi, involving a parallel universe in which, at some pivotal moment in our own past, some perfectly realistic event occurred to change history. Fast forward to some time around the present (or stay in the past) and watch a completely different world unfold to the one we know. This may involve events such as the Roman Empire not converting to Christianity (and/or not falling), China discovering the New World before Columbus, the Victorians establishing information technology of a Babbage/Lovelace design, the American Civil War or Second World War turning out differently than in our timeline, or such events.
Another tradition, which has recently become very popular, is a less “realistic” take on alternative histories, typified by Steampunk and certain flavours of Paranormal Romance. In the worlds created by this sort of story, a historical period is embued with magic or technology or alien intervention or other fantastic powers. In this tradition, there is usually less concern for realism than in the “What If” genre; not only historical events may be changed by the new interference, but social conventions, religious attitudes, language and idiom, etc., even in quite anachronistic ways. As might be expected, these stories tend to be more wild fantasy adventures, rather than clever nerdy historical explorations.
Tellingly, alternate history fiction writing is sometimes used as a technique for exploring history in academic writing (I have read more than one appendix to a serious book on ancient history imagining that Anthony and Cleopatra won at Actium, or the Greeks fielded weapons designed by Archimedes). Whether realistic or fantastic, alternative histories allow the writer to strip away the random, incidental details that are the skin of a story and look closely at the important bones and guts beneath: the social and political influences on and repercussions of events; the way people behave and interact; issues of sex, race, class and other inequalities that alien times and places can throw an unforgiving light upon.
So in steampunk stories women may play prominent roles in ways they never could in our Victorian period; a story set in a world without a Roman Empire might feature openly queer protagonists centuries before such was possible in our timeline; a different history of the New World could lead to Africans interacting with native Americans without the genocidal brutality inflicted on both sides by the descendants of Europeans. Or they might not, but because historical details become the choice of the writer, they can be highlighted in ways that tell us about our own inequalities, prejudices and crimes.
Good speculative fiction stories can cast light on our own world by showing a world that is better, or a world that is worse (or a world that is better in some ways and worse in others): either way it should show us how we need to improve our own.