Sometimes you need the exact facts about something in your story. Like, if you want to set a story in the real Paris or an imaginary version of it, you might need to know the name of the river that flows through it (the Loire), what the French call their money (the lire) and those sorts of practical details. It never hurts to check facts or tiny details. For example, I did some research about ocean currents when I was writing The Travelling Restaurant. After all, I figured that sort of thing would be true whether it was the real world or a fantasy.
Other times, doing some research can help your fantasy ideas get bigger and better. When I was writing The Queen and the Nobody Boy I wanted an unusual flying vehicle so I looked on the internet for the history of air travel. I learned that one early inventor thought that a plane would never be able to get off the ground so he imagined it being attached to a tower. People would climb up the tower into the plane, then the engines would start and off they’d go. I used that information as a basis for the wind-train that Hodie and Sibilla use to escape the Um’Binnians (except it gets them into more trouble). For The Volume of Possible Endings I wanted to have the first submarine built in Fontania. So I looked up the history of submarines and found heaps of fascinating stuff that happened in our own real world. For instance, centuries ago someone invented a submarine that used oars – underwater! It wasn’t a great triumph.
The not-so-annoying answer to today’s question is: no matter what you’re writing, it doesn’t hurt to find out what is possible and use the facts however you like to help your own story.