The public_ftp folder is used for storing files which are available with anonymous FTP.
You cannot use anonymous FTP with a Shared or Rseller hosting plan. You may with any Uber Plan.
Anonymous FTP users can view and download files if you have an Uber Hosting 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.