why ceilometer compute start like this way "sg libvirtd '/usr/local/bin/ceilometer-polling --polling-namespaces compute --config-file /etc/ceilometer/ceilometer.conf' "

I have a question about the ceilometer command line.
why it start like this way " sg libvirtd '/usr/local/bin/ceilometer-polling --polling-namespaces compute --config-file /etc/ceilometer/ceilometer.conf'  "
rather than "/usr/local/bin/ceilometer-polling --polling-namespaces compute --config-file /etc/ceilometer/ceilometer.conf"