It can be useful to bypass the userland sync(2) during reboot(8) while still allow the kernel sync to occur.
An example of this is when rebooting with disconnected iSCSI sessions which would otherwise cause the reboot to hang on BIOs that will never complete.
Add the -N option to reboot etc that allows the userland syncs to be skipped but still process the kernel sync during the reboot(2) syscall.