You know that line in book descriptions? "Everything changes when..."
The event that changes things for the character is the inciting incident. It's literally the event that "incites" the character to set the goal which carries the story.
And if the goal is your story's point, the inciting incident is then the catalyst that sparks off the story in earnest.
If you think about it from the reader's point of view, the story's goal doesn't exist until the inciting incident occurred. So the introduction has no direction; it's only an introduction. Direction only happens when the character says (directly or indirectly), "This is what I want to do." After that, the story is about whether or not that thing is achieved.
But it can't happen if there isn't some spark that makes the character set out on their journey in the first place.
For this reason, it's a good idea to have the inciting incident occur as soon as possible. Some people say within the first third of your story, but I personally think that's too long, unless your story has a slower pace. Others say you must start in medias res, and that the inciting incident has to happen in the first chapter. Which I say is too fast for most genres outside of mysteries (where the incident in question is someone dying or something being stolen) or a thriller. Personal experience says that most of my stories work best with a proper character intro, and the inciting incident occurring somewhere in the first fifth of a book. But that's because I prefer to emphasize my character arcs. Putting the inciting incident at around 10k words in (assuming I have a 50k book) gives me time to show the readers who the characters are before the inciting incident changes things, which I feel gives those changes more of an impact.
That said, I tend to personally leave it up to the story I'm writing, for the inciting incident to happen when it's ready to happen.
So why the spiel about where to put it, then?
Because a surprising amount of writers feel like their story is dragging half-way into the book and they can't tell why.
Often, the reason is that they've written half a novel's worth of words, but nothing's happened yet. So it's basically a half a novel of waffling around with no direction and no visible point. Because nothing happened to make the character decide to do something. And as such, nothing is done.
If you know what the inciting incident is supposed to be, you'll also know if it hasn't happened yet, and so you can make sure it does happen and soon enough to keep your story from lagging.
Do you pre-plan your inciting incidents? Do you prefer inciting incidents to happen right at the beginning, or at a later point in the story?