Other NPCs....

Not every NPC buys, sells, or even offers quests. Sometimes an NPC might be part of a quest or mission objective. For example, an NPC's job could be to pass on crucial information or fight at your side against a dire foe, or act as a temporary ally you must protect at all costs. Some NPCs are essentially part of the environment (like guards, town criers, or students) or become such when the quests they assign to you have been completed. Others might provide you with a bit of information about the current region, including history, current conditions, and local enemy activity. http://www.guildwars.com/gameplay/exploring/townlandscape/