In a rigid hierarchical society such as the Emerald Empire, knowledge of appropriate conduct is essential for all but the most jaded and uncourth bushi. In many ways a proper code of etiquette is a defense for samurai, who can use their knowledge of how one should behave in a court setting as a shield against the manipulation of others. It can also allow them to avoid embarrassing faux pas by ensuring that they know to whom any particular concern should be addressed in the vast and often confusing Imperial bureaucracy, as well as the lesser bureaucracies maintained by the Great Clans.
Etiquette is essentially the "social defense" Skill of honorable characters in Rokugan's courts. When someone tries to tempt, manipulate, or otherwise influence you (typically with skills like Courtier or Temptation), Etiquette is usually used in the ensuing Contested Roll, most often with the Courtesy Emphasis. Courtesy also allow you to offer an opinion without risking offense (at the TN chosen by the GM) or to remember important social mores that might otherwise be forgotten (TN of anywhere from 5 to 20, depending on obscurity). The Bureaucracy Emphasis is used when trying to maneuver through governmental channels appropriately, such as trying to arrange a meeting with someone of high Status (the Imperial bureaucracy is notoriously difficult to navigate). Conversation allows you to determine an appropriate, non-offensive topic for discussion (TN 15+).