Multi-FTP option allows the creation of FTP accounts which you give access to different resources stored on your server.
It enables you to give server access a third party (friends or clients) and allows them to manage the files via FTP. This person will manage his/her files via private FTP access, the access to other parts of your hosting space will be denied.
A service similar to a dedicated server
If you are a webmaster, this service enables you to offer hosting services to your customers. You don't have to manage the server yourself!