Or you're doing research on general climates because deserts fucking suck.
Or you're reviewing all the gore you've ever read and have to make sure you get the muscle structure of a person right.
Or you look up how muggers mug people successfully for reasons.
Or you check on how love works.
Or you gotta get the details of cooking juuuuuust right.
Or you have to look at the end of Neon Genesis Evangellion to find the right words to describe a horror scene.
Or you have to fall back on the stupid amount of dragon anatomy and psychology you know to determine the actions and reactions of other characters.

This is all in one book by the way.

I've been researching military formations, what a person could do to be dishonourably discharged, and the ethics of combat euthanasia. It does make the story more believable if you think about every single facet of the story, even if you don't explicitly write it down.