All native Unix filesystems do this (well...at least since V7; I don't remember about V6 or before). It's so the root user (and the kernel, and applications running as root, e.g. logging) would have some amount of disk space to work with if needed. Much more important in the days when typical servers had less than 100MB of disk.

The default is (usually) 5%. In these more bountiful days of multi-terabyte disks I generally knock that down to 1%.