Immutable backups

An immutable backup cannot be changed or deleted for a fixed period, by anybody, including whoever holds the administrator account.

That last part is the point. Once an attacker has full control, every protection that an administrator could switch off is one they can switch off. A backup that literally cannot be deleted until a set date removes their easiest move, which is to destroy the copies first and then ask for money. It is one of the few controls that keeps working after the attacker has won everywhere else.