Most NPCs serve a purpose. They can provide various services: some heal the player and his minions, repair the player's items, trade with the player or provide gambling services. In addition, most quests are obtained from NPC's. Some NPC's merely have a decorative goal to simulate crowdedness; players usually can't interact with them.
|NPCs in the Diablo series of games|
|Diablo II NPCs|
|NPCs from D&D Diablo|