The public_ftp folder is used for storing files which are available with anonymous FTP.
You cannot use anonymous FTP with a Basic or Economy plan.
Anonymous FTP users can view and download files if you have a Business or reseller plan. If you need to be able to upload to public_ftp, you can create an FTP account and set the home path as public_ftp/incoming, then anyone can use this newly created user name and password to upload to the public_ftp folder.
Anonymous FTP users can view, edit, download, and upload files if you have a dedicated plan.