The default monero.conf file depends on the existence of the folders `/var/log/monero/` and `/var/lib/monero/`.
This change makes sure systemd will create them, together with the proper permissions, if they don't exist.
`StateDirectory` can be considered an extra safety in case the user `monero` happens to have been created with specifying `/var/lib/monero/` as a home folder but without actually creating it.
Jared Monger