The Private File Storage area is where you can upload pictures, documents, and other files for easy access. For example, you could upload a scanned bill and give a Guest the permissions to view it and leave notes or respond.
Stored files can be completely private (only you can see them) or they can be made available on a Guest-by-Guest basis. You can even choose to allow anyone with internet access to download or view the file without having to create a Guest account for them.
You can add comments to each file, and a full file history is kept indicating every time the file is viewed, downloaded, or when comments are added to it by you or a Guest (if you've given them permission to add comments).