not sure if customers local roundcube instances are included in website backups, don't use them.
the global webmail site definitely isn't.
it should be.
in the database:
simple things like last login, failed login are useful for troubleshooting / security.
then there's client data... contacts...
calendars (if calendar plugin installed)
plugin files/folders/configs...
all of those can be lost if there's a server problem..
they should be automatically backed up regularly
