I tend to write stories that ease into the fantastical elements. Any urban or rural fantasy starts in the world as you and I know it – more or less – but that doesn’t lessen the need for worldbuilding.
In those cases, the worldbuilding is more subtle than, say, a sword-and-sorcery-type fantasy. However, in all situations, it’s important to create a multi-dimensional world that is consistent and true to its own created logic. A flat world is not believable and readers will poke holes in it, while inconsistencies will jolt readers out of the story – perhaps enough so that they won’t want to go back in.
My mother-in-law tends to equate it to a math class she had where the students changed a single rule and then did proofs to see the cascading effect on other rules and formulas.
That might have been a math class I actually enjoyed.
The point is: change one thing and pay attention to the butterfly effect. Maybe dragons fought in the Napoleonic wars, like Naomi Novik’s Temeraire books. Even small, seemingly insignificant details can have a ripple effect.
Sometimes in creating a just-sideways-from-here world, we don’t have the knowledge or perspective to follow those changes as far as we want to or should. I recently stumbled across this wonderful site that allows you to ask questions – often hypothetical – and have an entire community answer.
I haven’t yet used it myself, but I hope it’s as useful a worldbuilding tool as it could be. If any of you use it, please let us know how it works out!