Actually that feature is there so advanced users and users without Internet access can import and export keys, and also feel safe about it. You still need Premium to share a file key at this time, even if manually imported. We should definitely consider making it possible to do so in the Free version. Thanks for pointing this out.
The use of the function is to do what the server otherwise does, when you share the key to a file (we don’t share actual files) by typing in someones email, what happens is we get the public key from our server and then use it to share the file key.