Great question! The My Folder feature is available for all users, despite if they have access to save_content. An Admin user can save content to that user’s Folder for them which they would be able to access there. We also have this feature if you decide to upgrade your Viewer Users to a Standard User, where they would be able to save content and Explore.
A Viewer User is able to save content to a board because boards are a way to curate content - it is essentially creating a link for your other dashboards and Looks.