Conflict and peacemaking, setting or altering territorial boundaries, regulating trade, resolving disputes of all kinds, the protection of citizens abroad, protection of investments and protection from exploitation, promises of cooperation and mutual assistance—most forms of diplomacy (and even the need for diplomacy at all) are predicated on close contact and rapid communication. How do you conduct interstellar diplomacy without FTL travel, or at least FTL communication? Why would you want to?