Literary nonfiction is the closest thing to a written document. Take for example you were in the Renaissance era, where new art, and theries, and styles were being developed. You, a young novelist, were to write about what you saw. This written piece that you've written can be used to change history. How? Because you can write about, how it was developed, how it was created, the smaller details that we overlook.
Try and picture yourself now. You see a new historic site being built. And you wrote about it. 200 years into the future an architecture found what you wrote and used it to help change the style of future buildings.
Bottom line... it helps change, impact and inform us of what we weren't able to see.
Make sense?