Whereas Email permits exchange between equals. FTP allows you to fetch a (copy of a) file from a repository. Leaving aside the question of payment, a mail-order catalogue company might make a passable analogy.
FTP can be used as a private exchange mechanism but most encounters will be by "anonymous-FTP" whereby a repository is on open acess to the world.
For example - a demonstration copy of the genealogy package "Pedigree" is available from my FP repository on one of Oxford University's computers (sable.ox.ac.uk) in the directory (pub/users/malcolm/genealogy). Lets have a look at it ftp://sable.ox.ac.uk/pub/users/xzdg/malcolm/genealogy
Now to Usenet