There’s a whole world inside each of our characters. I think that world is what often brings us to fiction in the first place. I think written work lets us, as readers, inside a character’s head in a way no other method of storytelling every really does. In movies or theater, we see a character navigating through a problem, but we often don’t hear directly from those characters. We get dialogue, sure, but we’re not inside their heads, and we’re not hearing their thoughts.
Not like we do in fiction, anyway. In fiction, we do get to hear characters’ thoughts. I think it’s why we so often feel so close to fictional characters, why flipping the last page feels like losing a friend.
I often think the best worldbuilding, then, is the worldbuilding that gives us the setting and surroundings through the eyes of a character. Each time it rains or a character meets someone new or goes to a new location, we get closer to that character if we can see all of the settings and weather and people through their eyes. That way, the outer world that surrounds the main character isn’t backdrop, it helps the reader understand who they are—their wants, fears, motivation. If we as readers know how a book’s main character views the others in his / her life, how they view social structures, how they view their own place in the world, we understand why they’re acting a certain way. In fact, we might even begin to empathize to such an extent that we root for them when they’re acting bizarrely or selfishly—because we so thoroughly understand where they’re coming from.