From Ernest Renan, a French 19th-century philosopher:
Forgetting, and I would say even historical error, is an essential element in the creation of a nation, and that is why the progress of historical studies is often a danger for the nation itself.
Of course, the observation extends to other communities, not just states and nations. Kwame Anthony Appiah, in whose recent book (Experiments in Ethics) the Renan quote appears, added this as a commentary.
Offering an account of the past … is in part a way of justifying a contemporary practice. And once we have a stake in a practice, we shall be tempted to invent a past that supports it.
I think this gets at why historians have tried so hard to insulate historical writing from politics and bias … and why they so regularly fail. If I had more time I’d attempt a discussion of how this plays out in the writing of Mormon history. Perhaps some obliging reader can take a stab at it.