Let's assume you are the chairperson in a student organization and you would like to give someone else access to updating your website files. Obviously you don't want the person to have full access. Only access to the edit the website files, for this specific reason you can extra an extra FTP-user. This user will have access to the web files but cannot login into our web control panel.
Set a username and password and note the directory it will default to the username and thus will not be able to access the main website files. Remove the username from the directory to fix this. You can now connect to the FTP-server using SPECIFIED_USERNAME@DOMAIN_NAME. See How do I upload files to my website? for more information on how to connect via FTP.