Ah, the part I love the most both as a writer and as a reader. It is the practice of making the readers feel that the world exists outside of the plot, that background characters have lives outside of applauding at the return of the hero and that the beliefs aren’t just there to service the plot. A well built world helps the plot more than most people understand. It helps the reader be drawn into your world, making them attentive to every bit of your plot and when the readers are attentive towards your plot then readers do free advertisement on sites like tumblr and Facebook.
Now for an example of a well built world:- the lord of the rings trilogy. The characters refer to myths and legends in lord of the rings that may only exist in that world in a way that the people would really talk about in the real life about our myths and legends. It takes a fair few reads of the whole series to be able to comprehend the beliefs of the people present. On top of that, cities are built with their own unique style. It feels like it is a living breathing world.
Now you might say that it only helps fantasy genre but I would argue you every day of the week for that claim. While it certainly helps high fantasy and si-fi the most, it still is an integral part of the experience of other genres. In those novels, it may even be called atmosphere that the author builds around the story.
a good example of a thriller with a well built world: angels and demons. (spoiler): when Rome is about to blow up at the end of the book due to the anti-matter explosion, I feel anxious because I care for Dan Brown’s vision of Rome. He has made us care for its people, its symbols and Its history. This makes for a thrilling scene towards the end. By contrast, I wasn’t as sucked into his later books (except da vinci code) because we did not ever feel the stakes of failure, we did not care for the cities they were based in because the action kept moving every time we got even a little comfortable with the setting and it just was a poorly made world in general.
In horror, it may be the difference between a comedy and a genuinely scary scene. In romantic novels, it may be the driving force of the plot. In a thriller, it might build the suspense and tension in each scene.
Bottom line: good world building is integral to story-telling. In fact it is done every time we tell a story because every time, we do build a world of sorts when we give context and without context, a story just isn’t good enough.