i think the OP is confused as to the nature of roundcube.
Roundcube is NOT an all encompassing email system.
that is to say, EMAIL users are not created and managed in the roundcube system.
Roundcube connects to an email server and presents mail to a user who logs in. JUST like a mail client, aka thunderbird, outlook
the reason that roundcube creates and manages new users is so that users using roundcube can store a contacts list, and probably store other preferences. JUST like thunderbird creates a user profile on your computer to store preferences and contact lists. the contact list that roundcube manages is obviously only accessible when using roundcube's interface.
since roundcube serves the same purpose as a mail client like thunderbird, but instead of being on the machine you are using and is hosted on a web server for all people to access... mails aren't actually pulled off of the server and into roundcube... you just use roundcube to view your mails, manage them, and send new ones.
in short, its all kinda like the voicemail system for phones. you can use any phone (email client: thunderbird, outlook) to access your voice mails (emails) by dialing your voicemail provider (accessing your email server), and know that roundcube is a phone (email client) not a voicemail service (email server).
List info: http://lists.roundcube.net/users/