The man page of `tune2fs` gives you an explanation:
> Reserving some number of filesystem blocks for use by privileged processes is done to avoid filesystem fragmentation, and to allow system daemons, such as syslogd(8), to continue to function correctly after non-privileged processes are prevented from writing to the filesystem.
It also acts as a failsafe; if for some reason the normal users and their programs fill up the disk up to 100%, you might not even be able to login and/or sync files before deleting them. By reserving some blocks to `root`, the system ensures you can always correct the situation.
In practice, 5% is an old default and may be too much if your hard drive is big enough. You can change that value using the previously mentioned `tune2fs` tool, but be sure to read its man page first!