This is the home directory of the System Administrator, 'root'.
This may be somewhat confusing ('root on root') but in former
days, '/' was root's home directory (hence the name of the
Administrator account). To keep things tidier, 'root' got his
own home directory. Why not in '/home'? Because '/home' is often
located on a different partition or even on another system and
would thus be inaccessible to 'root' when - for some reason -
only '/' is mounted. The FSSTND merely states that this is the
recommended location for the home directory of 'root'. It is
left up to the end user to determine the home directory of
'root'. However, the FSSTND also says that: If the home
directory of the root account is not stored on the root
partition it will be necessary to make certain it will default
to / if it can not be located. We recommend against using the
root account for tasks that can be performed as an unprivileged
user, and that it be used solely for system administration. For
this reason, we recommend that subdirec