Archives, because they have been in existence since the time of Babylon and Sumeria, have typically been institutions that act as catch-alls. If you put all the archival collections of the world in one place, it would probably mostly resemble a junk drawer: a lot of things you need sometimes, some things you have only needed once, and some things that without which you couldn’t live your day-to-day life. Archives try to collect both things that will see a lot of use, and things that might someday be useful, to understanding the history of a community.
Each archives should, ideally, be focused on collecting in one particular area. Some archives are for businesses, or governments. These collect business records of the corporation or agency and make sure there is an “official record” of that entity. Other archives collect based on geography; a historical society is a good example of this. The Galveston Historical Society or the New York Historical Society are dedicated to preserving their own place’s events and people. Sometimes this will include the records of businesses or governments, but it is geographically focused nonetheless. There are also archives which are for a certain topic–comic book archives, World War II archives, or Hollywood film archives.
What all these archives have in common, though, is that they are collecting “the thing itself.” In a Civil War archives, for instance, the archivists do not collect books or movies about the Civil War (these secondary sources are held in libraries). They collect letters from Civil War soldiers, newspaper articles from 1863, actual uniforms, early photographs taken on the scene. In the case of a government archives, the archivists will keep the actual letters signed by governors, the actual drafts of legislative bills, the tax forms themselves. They will not keep a book about a senator, but would rather keep the original manuscript of the senator’s own diary.
Archives are devoted to creating access to the original. An archivist will help their patrons to understand the original, to use it and to find other original things that relate. But because the story of history is always changing, an archives doesn’t try to give their community or their patrons a story, like museums do. They only give the documents, photos, and objects that have been created by the community, and let others try to find a story from them.
For more on what archivists do and why, you can also look at the Society of American Archivists Core Values Statement and Code of Ethics.