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 may be significantly cheaper to pop and have lower upkeep than Warlords who are more useful directing battles in the field. It is possible that Warlords provide a larger efficiency boost to their city than a courtier (to make up for the higher expense) but may have questionable strategic valuesummer-updates-035 when the city is not under siege.
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.