“Representing reality — whether “real” reality or a fictional one — is simply one way of telling a story, just one house in the city of fiction.”
This is the key, I think. Worldbuilding isn’t an end in and of itself, but rather a tool for telling a story — particularly a tool for establishing consistency (but here again, “consistency” is also not its own end, but is a tool to enhance one thing or another in the story itself.
My favorite example (and the type of worldbuilding-related plot device I find very frequently executed poorly) is making characters appear powerful. Lots of stories I’ve read describe (or show, if in visual media) a character’s strength through direct comparison to another character, but seem to achieve this through a poor performance by the “lesser” character rather than a better performance by the stronger character. This often winds up feeling like a plot device rather than an event — as if the audience reaction should’ve been “wow, they’re strong” but is instead “we get it, they’re strong.”
I find that this is due to a lack of consistency in the way the power is displayed — the writers’ portrayals of the characters’ feats seem to float according to what’s necessary for the plot, and the audience just has to take their word that the character relationship is as shown. This kind of consistency would be much easier to write with some kind of rule set or foundation defining what the characters can do in the world. The audience doesn’t necessarily need to be specifically aware of what these rules are, but if the characters are written according to those rules, the rules can be inferred, and suddenly one character defeating another gains a lot of meaning without ever feeling like the author is saying “take my word for it.” The stories that do follow their own internal rules of this sort are very noticeable, just by how much more of an impact these kinds of comparisons have even without thinking directly about the worldbuilding information.
As I like to say, humans are pattern-finding machines. Our minds take all of our experiences with the world and distill it all into overarching concepts or rules of thumb. To us, the world is a system of these rules and principles — but we don’t experience them directly, we experience them through detail. Thus, writing a world should give detail from which we can infer a fictional world’s overarching principles. (I think this is closely related in concept to “show, don’t tell”)
That means that a writer must do some extent of worldbuilding in order to present the audience/reader with details that consistently infer underlying principles of their world just like the details of the real world do.
I think this is where my definition of “worldbuilding” differs from what may be conventional: I’ve always seen “worldbuilding” as the process of fleshing out a fictional world to any given level of detail — a wholly separate activity from then writing a story that takes place in that world. “Worldbuilding” sometimes seems to refer to the bits in the story that describe or imply the world, and under that definition, I totally agree: you don’t need insane detail explicitly stated within the story. The consistency provided by detailing a world is just a tool to writing a compelling story, and isn’t an end unto itself.