This lets us see how history was changed to fit then-prevailing themes and ideas.
A good example is the Mesoamerican geography.
For example, people cite this one to me, from History of the Church, Volume 5, June 25, p. 44, online here.
Saturday, 25.- Transacted business with Brother Hunter, and Mr. Babbitt, and sat for a drawing of my profile to be placed on a lithograph of the map of the city of Nauvoo.
Messrs. Stephens and Catherwood have succeeded in collecting in the interior of America a large amount of relics of the Nephites, or the ancient inhabitants of America treated of in the Book of Mormon, which relics have recently been landed in New York.
Here's the actual journal entry from the Joseph Smith Papers here. This was the contemporaneous writing:
Saturday 25 Transacted Business with Bro. [Edward] Hunter. Mr Babbit [Almon Babbitt]. & set for the drawing of his profile. for Lithographing on city chart.
Yep, that's it. All the stuff about Stephens and Catherwood was added later, after Joseph's death. It was never in Joseph's Journal, but someone reading History of the Church would believe Joseph recorded this in his journal.
And then that person would send the passage to me to show that instead of a North American setting, Joseph explicitly connected the Book of Mormon with the Stephens book.
And so it continues. People really have no idea how pervasive and ingrained this Mesoamerican theory is. But we'll keep pointing these things out so you don't have to be confused by changes to Church history.