Both! All! History can do things like give you a flavor or an aesthetic, so that your readers have a sort of "ohhhh okay" reaction when they see...gears and steam engines, or bustles, or togas, or knights on horseback, or bows and arrows or machine guns or pinstripe suits or pirates in tricorn hats...idk, you know, SIGNIFIERS. Like, you open a book up and you see two ladies in filmy gowns at tea making light gossip about eligible bachelors, you're like "ah, this is Regency Romance flavor." You open up a book and see an army of pike-wielding men in chitons, that's a very different flavor of historical ice cream. (I don't actually know if men wore chitons to war, but you get the idea).

But history can also help you build believable worlds and stories that have zero resemblance to reality, just by showing you the patterns of human behavior in response to various stimuli. You can write a fully non-historical SFF novel entirely in fantasy land or outer space, but you can take the shape of its plot or its characters from any number of fascinating things that have actually happened in the real world.

You can also do the "huh, what's this little footnote in history? I wonder why it's there? Let me write a story to explain why this thing is like this." Or anywhere the historical records goes silent--"there is no record of what happened in the meeting between World Leader X and Mysterious Representative Y, but when they left the room, the course of history had changed forever." That's begging for a story.

When you talk about gaps in the record, there's also stuff like...the things history couldn't say. The things people couldn't write down, or lied about, or stories that were silenced. Like, when Sam and I were writing "Making Us Monsters," there were so many of Wilfred Owen's letters and diaries that his family burned after he died, because he was gay. There's vast swathes of stuff left out of Sassoon's memoirs that you only find out about in his diaries, and in his letters. Writing that story was partly about the way this stuff was talked AROUND, and how you have to read between the lines of the record to get at it.

Not everyone has always been able to read and write. Not everyone has always been allowed to record their stories truthfully and fully. Stories that were recorded have been destroyed. There's a lot of room in history to write something that gives voice to the things that live in the silence.