This allows you to offer third-party access to your web hosting plan (a friend if you are an individual, or customers, if you are a reseller), and leave them to manage their files via their own FTP access, without allowing them access to other areas of your web hosting plan.
The multi-FTP option enables different FTP accounts to be created, allowing each one to manage different parts of the web server.
A service similar to a dedicated server
If you're a webmaster, the service enables you to offer hosting services to your customers, without having to manage the server yourself!
Various users of FTP sharing.
Add more users to your FTP by creating "mini FTPs", which users can manage independently.
Your visitors will visit the websites without knowing that they are sharing the same space.