One big advantage for pure mailserver configurations is that the SASL layer can use a password file of its own, so that mail users need not have a system account.
The command saslpasswd (in the default install this is located in /usr/cyrus/bin) is used to add accounts (saslpasswd -c <username>) and change passwords (saslpasswd <username>). The command stores/changes accounts in the file /etc/sasldb.db which must be readable by the user running the Cyrus sofware.