In Linux (and whatever else; don't fuck with me!), this (optional) directory is really just another user directory with needed configuration files. It is the equivalent of all the user directories within /home. The difference between root and other users is simply that root == god and shouldn't really be used for anything except system administration and maintenance.

There is a very good reason for root's home directory to be outside of /home. On a large number of Unix computers, /home is mounted from a network drive, through NFS or something similar. If there's some problem with the network, either during boot or normal operation, /home tends to vanish as soon as whichever network service it was mounted through notices that it's awfully quiet out there.

Now, in order to fix most network problems, one needs to log in as root. In order to log in as root, root needs to have a readable home directory. If root's home is /home/root and /home is missing in action, this will fail. On some Unices, this can be a very bad thing.