Well, they are like fuel for the imagination. A good fictional world in a book can transport a reader to places they've never been and times that don't exist. The characters' abilities, the rules of magic or technology in that world, and the overarching plot all play a role. Take the wizarding world of Harry Potter as an example. The spells, the magical creatures, and Hogwarts itself have inspired countless fans to imagine themselves as wizards, flying on broomsticks and dueling with dark wizards. It's not just about seeing what the author has created but also about using that as a springboard to jump into our own imaginative flights of fancy.
They expand it greatly. When we read about fictional worlds, we are exposed to new concepts, unique cultures, and extraordinary creatures. This gives our minds new things to think about and visualize, making our imagination more vivid and far - reaching.