To add to rather than substitute for what has already been written in these answers: it is worth pointing out why Worldbuilding can be particularly helpful for fiction-writing (whatever the medium).
If you think about how our own world works--"world" here meaning anything from physical universe to local town neighborhood--you might notice the omission of information from conversation. There are lots of things that are so obvious and known that nobody bothers to mention them.
- Elementary mechanics
- Calendar and timekeeping systems
- Many foods
- Language structures
- Mainstream religious norms
- Basics of transportation
- Elementary sexual politics and dynamics
- Elementary racial politics and dynamics
This is not to say, of course, that these things are never discussed or commented upon. Lots of people love to get into lots of detail about food, for example. But there are basic assumptions that don't get discussed: onions and tomatoes are edible foods, for example. Why wouldn't they be? So nobody comments on it.
If you're writing a story set in an alternate world, be it another culture, another time, another universe, one very common and unfortunate tendency is to explain things that don't need explanation, and whose explanation in the story's narrative can be frame-breaking.
For example, if characters in your story start having long conversations explaining how timekeeping works (the number of hours in the day, etc.), that's going to seem strange to the reader, because it jars against the sense of ordinary normality. Many times, the writer has done some Worldbuilding, and figured out some nifty alien system for timekeeping or whatever, and as a writer she badly wants to share it with her readers. But it's going to be tricky to pull off, because it's hard to imagine why such things would get discussed.
One thing that the Worldbuilding process can help with is to move from this kind of "idea-dump" to a fuller integration. If you as a writer know the world very well, and you understand how all these funny alien elements of it work together smoothly, you don't have to explain everything. You can instead try to make your story seem as though it were written by someone living in your alternate universe. Because the story now includes various passing references, terms, allusions, and so forth, all perfectly ordinary and trivial to the implied or explicit narrator but alien to the reader, we get a sense of a real world. And if those passing bits actually link up to one another, not because they're important to the plot but because they're part of a complete world, the whole thing comes to seem rich and vibrant.
Tolkien's Middle Earth is the most famous and obvious example. His ability to have characters remark blandly that it's blacker than the cats of Queen Beruthiel works beautifully: it's just the sort of thing people do say, and it doesn't really mean anything, but we get a strong sense of a world in which there are a great many stories and such that we aren't being told because they don't happen to be relevant. The story of the War of the Ring just gives us a sort of slice through a very complex world, and we end up learning far less about that world than we might appear to at first glance. This is part of why Middle Earth seems so real.
But Tolkien did this the long way: he actually invented all the stuff to which people refer. That's great if you want to spend your life at it, but it isn't necessary. You can do it by simply tracing out a thin surface and making sure the various pieces weave smoothly.
Of course, to do that coherently does require that you know everything about everything. Supposing you don't, you can ask here. You know all about the languages, let's say, but although you know that this group uses bows made from giant insect chitin, it occurs to you that you don't know anything about insects. So you explain here that you've got these people who use these bows, and you want them to have 60-pound draws, and you wonder whether that means you have to use 6-foot wings or legs or what, and so on. And people here who do know about insects and archery and engineering will try to help you get the information you need so that your characters can use their bows and refer glibly to giant cricket wings, and your readers will come to feel that it all makes sense because you, the writer, know enough to ensure that it does in fact make sense.