Exchange 2016 has 5 types of system mailboxes: Discovery Search (1 mbx), Arbitration (6 mbx), AuditLog (1 mbx), Public Folder (not created by default) and Monitoring (the number depends on the number of databases). All of them are necessary for full-fledged Exchange's work.
You can't delete the first mailbox database (or another one) until all system mailboxes migrate to a new database.
