The file element's purpose is mainly used when you have your own logic added or in a form for example.
It can be used in sections, but it won't return any value to the frontend, nor will it upload the selected files. It will only create a file input field in the cms which is not particulary usefull. If you want to have a user select a file from the media library, choose the media element instead.
The file element is especially usefull in forms. Files can be sent as attachment if mails are sent on form submission.