Cultural norms vary from location to location, so to truly engage in a book, we need to know whether we're in Dubai or Detroit. We're going to have a different visual in mind if we hear that a character goes to the market Paris or Los Angeles.
Although this can color the way we read when working with fiction, it is especially important when working with any of the non-fiction genres.
A quick diversion into a genre discussion. We all know that there are two main camps of writing and publishing: Fiction and Nonfiction. And we know that fiction is broken up into things like Romance, Science Fiction, Westerns, Fantasy. And each of those are broken into smaller pieces (such as Regency Romance, Modern Romance, LGBT Romance, Futuristic Romance, etc.). (You can find the full list of codes from the Book Industry Study Group, here, if you want to poke around.) OH - and I should probably mention that this doesn't even begin to take into account the listings of genres and subgenres and sub-subgenres that appear in classification systems on sites like Amazon.com.
On the Nonfiction side, you'll find exponentially more options, because each is broken down into its field, first, and then goes into specific sub-groupings. (Again, the link above will show you how many there are.)
One of the very important things to note, here, is that Fiction and Nonfiction are separate. The intent is that authors can tell their readers whether what they are writing about it truth or lies. Though this does get a little hazy in the Memoir area, where books labelled Autobiography and Biography tend to be held to a tighter non-fiction standard than do books labelled as Memoirs (with the idea that memoirs are stories told through recollection, while the others are told through researched facts).
Did you catch that last bit about Memoirs? Basically, they can be the same as that one story that your uncle tells about jumping out of the barn's hay loft every winter into the snowbank below. Only every year when you hear the story the snowbank is a different height - and when you ask your other uncle about it he's quick to point out that it only happened one year - and it was a first-floor window, not the second-story. The idea is that the memoir is based on facts, which might just be a bit fuzzy. And - assuming the author admits that at the outset - that's pretty much okay with most readers.
But let's get back to why facts - even fuzzy ones - are incredibly important to writing. I'm currently working on two very different memoirs. Though both (at least in part) cover the same time period (basically 1935-1965), one is set in the American Midwest, while the other is set in Germany. And, while they both discuss both urban and rural life, those lifestyles have very little in common. (Only one of them discusses air raid sirens, for instance.) If I didn't know going into them where they were set, that would make a huge difference to the story I was reading.
Each of the stories is rooted in very specific facts - pieces of information that can be proven to be true - though. For instance: We can look at a history book and find out when the majority of the Allied bombing attacks on Berlin took place. We can pull a yearbook from the Hamline University library and find out when a specific student was attending.
These are facts. They cannot be disputed, because they are known, verifiable quantities - and they make the story much more grounded and much more tangible to its readers.
Of course, we are living in "interesting" times, when facts are often disputed. There are people who believe anything they find on the Internet - just as they might believe any gossip they hear at their local coffee shop - and they insist that it's true even though it can't be verified in any way.
Consider a cook book - one of the ultimate "verifiable, nonfiction" book types. If the recipe says that it will feed 40 people, but when you make it it only has enough for 10, that's a problem. You're probably going to tell people about that issue every time you talk about the recipe - even if the flavors were great and it was simple and quick to make. And the 30 people who didn't get to eat will probably mention it, too.
Let's say that you're reading a piece of literature (fiction or nonfiction) and someone who says she was born in 1930 also says that her older brother was born in 1937. You instinctively know that one of those details is wrong. (Don't you? I mean... unless there is some time travel aspect going on.) And, most likely, your reading of the rest of the book will be colored by that realization. You'll wonder for the rest of the time which of the two dates is wrong - and whether you can "trust" the narrator. You may even end up going online and posting a bad review on Amazon or Goodreads because of that messed-up detail.
A fact-checking sidenote: Wikipedia, though much better vetted than it used to be, should not be your main fact-checking source. It is a constantly evolving document, with people going in and altering it almost as fast as it can be re-uploaded. Yes, it's a good place to verify things you think you already know, but consider it to be the same as asking a dinner party for the answer to your question. The longest-lasting response may not be the right one - it might simply be the loudest.
So, if you know that you'd go through all of that as a reader, that should probably also make it obvious that you should do your utmost to avoid those same situations in your own writing.
Check your details. Make sure that your character's three-month trip doesn't go from April to August. Make sure that the caption you put beneath an image saying "three siblings and the bear" doesn't accompany a photo of two kids and a peacock. Verify your details to the best of your ability - and if (as in the case of many memoirs) you can't verify them completely, let your readers know.
After all, we've all read the reviews by people who get great glee in pointing out a book's flaws - let's try to make sure that doesn't happen to you.
**If you want to have an editor do a fact check on your book, you'll probably end up paying more than if he or she is only working on grammar and punctuation. Why? Because it takes more time - much more time. After all, if it were a quick and easy task to do, every author would already be doing it.