Document files are the paperwork needed to keep the project on track. A landlord approval letter is an example of a document that would be stored here and kept for future reference.
Permits in hand? Now they can be scanned and stored here. If the city has an internal error and three years later, they call you to say that a permit was never signed off, you can pull the documentation and send it right over.
Artwork that the client gives you, survey notes, and all other important documents are stored here for all users to access and use. Good record keeping is your ally.