An FTP server is a very good thing: it allows you to share files with others with virtually no restrictions so that the data can be accessed by several people at the same time.
The advantage of FTP is that the other party can choose when to download, while on the server-side, for example, you can control who can see what and who can download what through the permissions settings.
The FTP server is very useful when you need to send large data files to a friend (say, family photos in their original size, holiday videos, or series). The good news is that you don’t really need anything extra: there are free programs that allow anyone to set up an FPT server on their home PC (even notebook) in a flash, with just a few clicks.