The securelevel mechanism is intended to allow protecting the persistence of code and data on the system, or a subset thereof, from modification, even by the super-user by providing convenient means of “locking down” a system to a degree suited to its environment.

Source: NetBSD 6.1.5 – man page for secmodel_securelevel (netbsd section 9) – Unix & Linux Commands

Highly secure mode may seem Draconian, but is intended as a last line of defence should the super-user account be compromised. Its effects preclude circumvention of file flags by direct modification of a raw disk device, or erasure of a file system by means of newfs(8). Further, it can limit the potential damage of a compromised “firewall” by prohibiting the modification of packet filter rules. Preventing the system clock from being set backwards aids in post-mortem analysis and helps ensure the integrity of logs. Precision timekeeping is not affected because the clock may still be slowed.