Roundcube is a client application. Compare it with Thunderbird. If you create an account in Thunderbird it will need informations about the IMAP and SMTP server to connect to.
Roundcube creates its users automatically if ...
#1 configured:
main.inc.php
// automatically create a new RoundCube user when log-in the first time.
// a new user will be created once the IMAP login succeeds.
// set to false if only registered users can use this service
$rcmail_config['auto_create_user'] = true;
#2-Connection to IMAP server backend is successful for this user.