Courtiers act as advisors, politicians, and diplomats, aiding those rulers who lack those abilities.
Courtiers may be able to manage cities in place of warlords. Don King's observation that using warlords instead of courtiers was "a risky extravagance" suggests that courtiers are significantly more productive and/or less expensive.
In this way they may act as a sort of friendly "A.I."; in some strategy games, the computer can handle basic maintenance for you.