What are the differentc *.secrets files needed for?
They are needed to store credentials when accessing other hosts and services. In details here is an (incomplete) list of files with a short explanation.
Used for authentication with the machine account to the master
Only Master, Backups, Slaves
For account “uid=join-backup,cn=users,$ldap_base”
Used by services/univention-samba/26univention-samba.inst to join the Samba DC