Note: During BYU Education Week, I'm posting some basic principles for new readers.
The New York Cumorah has never been more important than it is now. Not only are critics of the Book of Mormon claiming the book is 19th century fiction, but many faithful Latter-day Saints, forced by scholars to accept the Mesoamerican/two-Cumorahs theory (M2C) as the only possible interpretation of the text, no longer believe the Book of Mormon is authentic history.
It's easy to see why this is happening. Long ago, Joseph Smith explained that "if we start right it is very easy for us to go right all the time but if we start wrong it is hard to get right."
Joseph and Oliver made sure we could "start right" by explaining that the New York Cumorah was a fact. They could not have been more explicit.
Nevertheless, certain scholars have persuaded many Latter-day Saints to reject what Joseph and Oliver taught, making it difficult for us to "go right all the time" and "hard to get right."
There were two simple reasons why Joseph Smith and Oliver Cowdery explained that the Hill Cumorah of Mormon 6:6 was the same hill where Joseph found the plates.
#1. They visited the repository in the hill multiple times. This used to be well known by most Latter-day Saints. Brigham Young brought it up shortly before he died because he feared--correctly, it turns out--that the knowledge would be lost if he didn't discuss it publicly.
For an analysis of Brigham's statement about the repository, go to:
You can see some of the references to the repository at this link, which has a link to a paper on BYU's website:
The problem with that paper is it edits Brigham's talk to omit some key points, as you can see by looking at the analysis of Brigham's statement.
#2. They knew it was solid evidence of historicity. Mormonism Unvailed had claimed the Book of Mormon was a fictional account, based on Solomon Spalding's lost novel about the origins of the Moundbuilders in Ohio. https://archive.org/details/mormonismunvaile00howe/page/278/mode/2up
Joseph and Oliver denied they used Spalding's manuscript. That was enough for believers, but because that Spalding manuscript was never found (like the gold plates), they could not refute the claim by demonstrating that the Spalding novel was not the source of the Book of Mormon.
Consequently, in Letter VII, Oliver refuted the Spalding argument (and all other arguments based on the claim that the Book of Mormon is fiction) by relating the fact that a key location of Book of Mormon was definitely known: the New York Cumorah. In other words, he explained it was a fact that the hill from which Joseph retrieved the abridged plates in Moroni's stone box was the very Hill Cumorah of Mormon 6:6.
If you haven't read Letter VII, here is a link to Joseph Smith's own history:
By declaring the New York Cumorah was a fact, Oliver firmly established the historicity of the Book of Mormon. Without such a definite "pin in the map," critics can easily claim the book is fiction. With that definite "pin in the map" we can make sense of the text in the real world. IOW, with the New York Cumorah as a key to interpreting the text, we can reconcile the text with relevant scientific information available today.
For example, we can see that, based on relevant archaeology and anthropology, the population numbers in the text were accurate. There were never "millions" of Nephites. They did not build massive stone pyramids. They did not live among a much larger literate, culturally different, population that the text failed to mention. They did not live among volcanoes (which are never mentioned in the text). They lived along rivers and built ships.
The Nephite/Jaredite civilizations consisted of what we know today as Hopewell/Adena. This is why Joseph Smith referred to the plains of Ohio, Indiana and Illinois as the "plains of the Nephites." This is why he identified as Jaredite a site we know today as Adena, and he identified as Nephite a site we know today as Hopewell.
Now that many prominent LDS scholars have rejected Oliver's statements of fact (even though he was the Assistant President of the Church when he wrote Letter VII and even though he and Joseph had personally visited the repository in the hill), the strong defense of the historicity of the Book of Mormon has been removed. Consequently, believers are left with a cacophony of claims about the actual setting of Book of Mormon events. BYU teachers use a fantasy map to teach the Book of Mormon, conveying the unmistakable impression that the text describes a fictional setting.
The New York Cumorah, consistently and persistently taught by Church leaders starting with Joseph and Oliver, serves the same purpose today that it did in 1835 when Oliver published Letter VII. It refutes claims that the book is fiction and it serves as a guide to understanding the text.