For me as a writer, the best thing about writing SF and fantasy is this:
You can totally make stuff up.
If I write a modern novel set in Tanzania where people sit around in evening suits drinking Earl Grey tea and talking about what a jolly good show the fox hunt was, then reviewers will say ‘what a dumb-ass, people in Tanzania don’t act like that’. If I set it five hundred years in the future in a resurgent British empire, then no-one can complain because it’s a world I totally made up.
Obviously there are a limits, but this allows you to concentrate more on the story than how it’s told. Taking my part-written SF novel The Thing (yeah, I’m really bad at titles) as another example, it’s set in North America; if it was modern day, people would complain that the characters don’t act like Americans, but it’s set nearly four hundred and fifty years after the United States collapsed in a war, so the time between then and now is almost twice the time between the founding of the country and the present day… no-one can say they aren’t acting realistically because anything could have happened in the meantime.
Of course reality does intrude; in that case many of the geographical and economic factors which applied to the USA also apply to the new culture growing after the Cataclysm. But most of the past is made up, so as long as it’s believable within the context of the story no-one can say that it’s wrong.